Chapter 2. Creating an Addressing Plan for Fixed Length Mask Networks
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Chapter 2
Creating an Addressing Plan for Fixed Length Mask Networks
Introduction
Determine Addressing Requirements
Review Your Internetwork Design
How Many Subnets Do You Need?
How Many IP Addresses Are Needed in Each Subnet?
What about Growth?
Choose the Proper Mask
Consult the Tables
Use Unnumbered Interfaces
Ask for a Bigger Block of Addresses
Router Tricks
Use Subnet Zero
Obtain IP Addresses
From Your Organizations Network Manager
From Your ISP
From Your Internet Registry
Calculate Ranges of IP Addresses for Each Subnet
Doing It the Hard Way
Worksheets
Subnet Calculators
Allocate Addresses to Devices
Assigning Subnets
Assigning Device Addresses
Sequential Allocation
Reserved Addresses
Grow Towards the Middle
Document Your Work
Keeping Track of What Youve Done
Paper
Spreadsheets
Databases
In Any Case
Summary
FAQs
Exercises
Subnetting Tables
Class A Subnetting Table
Class B Subnetting Table
Class C Subnetting Table
Subnet Assignment Worksheet
Solutions in this chapter include:
Determining the number of addresses needed
Obtaining the correct size block of addresses
Choosing the correct mask to use
Allocating addresses to devices
Documenting what youve done
Using tools to ease the job
Introduction
Many organizations, especially smaller ones, use fixed mask addressing. Fixed mask addressing is easier to understand and simpler to implement than variable mask addressing. In fixed mask networks, every device uses the same mask and all subnets have the same number of available addressestheyre all the same size.
In Chapter 2 we learned about IP addresses and the basics of mask operation and subnetting. In this chapter, well detail the steps you need to take to assign appropriate IP addresses to those devices that need them. Well also show you some effective and surprisingly simple tools to make the job easier.
In Chapter 1, we established that routing and addressing are intimately linked. Your choice of routing protocols can affect your choice of mask. Of the popular routing protocols, RIP (version 1) and IGRP impose certain requirements on addressingall devices on all subnets must use the same mask. In other words, you are forced into a fixed-length-mask addressing plan. If you use RIP (version 2), OSPF, or EIGRP, then you can still choose to use the same mask for each subnet, but the protocols do not demand it.
Determine Addressing Requirements
When you need to develop an IP addressing plan, whether it is for fixed- or variably- subnetted networks, you have to start by determining exactly what your needs are. As you recall, IP addresses contain information that helps routers deliver datagrams to the proper destination networks or subnets. Since such a close relationship exists between IP addresses and their target network segments, you must be careful to determine the proper range of addresses for each network or subnet.
Review Your Internetwork Design
We start by reviewing our network documentation. If this is a newly designed IP network, youll need the design specifications. If the network has been in operation for some time, you can use the as built documentation. These specifications should include information such as:
The number and type of devices on each LAN segment
An indication of which of those devices need an IP address
The devices connecting the segments, for example: routers, bridges, and switches
For Managers Only
You do have a complete, updated, and available set of network design specifications and layouts, dont you?
How Many Subnets Do You Need?
As you review your design, identify and list each subnet, noting the number of IP addresses needed in each. Take a look at Figure 3.1.
Figure 3.1 Sample Network Layout.
One definition of a router is that it is a device that interconnects networks. Routers and layer-3 switches operate by forwarding packets from one network to another so that the packet gets one step closer to its final destination. Each interface on a router needs a unique IP address. Furthermore, each interfaces IP address must be belong to a different network or subnet. Put another way, each router interface defines a network or subnet. This last statement is the cause of much weepin and wailin on the part of IP network administrators.
Look again at Figure 3.1 in light of our routers configuration needs. Router1 has four interfacesone LAN interface and three WAN interfaces. Therefore Router1 needs four IP addresses, and each of those addresses needs to be in a different network or subnet. Now look at Router2. It has two interfacesa LAN interface and a WAN interface. Therefore, two addresses are needed, one in each of two networks or subnets. The same can be said for the other two branch office routers.
Lets tally what we have so far. The Headquarters router needs four addresses and each of the branch routers needs two, for a total of ten addresses. Does that mean that there are ten subnets? Look again: Router1 and Router2 are connected to the same subnet (labeled B in Figure 3.1). Router1 shares connections with Router3 and Router4 in the same way.
So we see a total of seven subnets: four are LANs and three are WAN connections. Do you need to allocate IP address ranges for all of them? In general, the answer is yes. As with most topics in the IT industry, the precise answer is more complicated than that.
How Many IP Addresses Are Needed in Each Subnet?
Now that you know how many different subnets (address ranges) you need, its time to determine, for each subnet, how many devices need addresses. The basic guideline here is that each interface that will be talking IP needs an IP address. Here are some examples:
Routers: one IP address per interface (see the next section for a discussion on unnumbered interfaces ).
Workstations: generally one address.
Servers: generally one address unless the server is multihomed (has more than one interface).
Printers: one address if they are communicating with a print server via IP, or if they have an integrated print server feature (like the HP JetDirect). If the printer is attached to the serial or parallel port of another device it does not need an IP address.
Bridges: normally bridges do not communicate using IP, so they do not need an address. However, if the bridge is managed using an SNMP-based network management system, it will need an address, because the data collection agent is acting as an IP host.
Hubs: same as bridges.
Layer-2 switches: same as bridges.
Layer-3 switches: same as routers.
In Table 3.1 you can see the number of various devices on each LAN of our sample organization.
| LAN | Devices |
| Headquarters | 20 workstations, 2 servers, 1 managed hub, 1 network-attached printer, 1 router |
| Morganton Branch | 11 workstations, 2 network-attached printers, 1 router |
| Lenoir Branch | 12 workstations, 1 router |
| Hickory Branch | 5 workstations, 1 server, 1 router |
Table 3.1 Devices in the Sample Network
Is the table complete? No. Whats missing? Remember that each router interface needs an IP address, too. Also, what about the WAN links?
Table 3.2 summarizes our actual needs, on a subnet-by-subnet basis.
| Subnet | IP Addresses |
| Headquarters | 25 |
| Morganton | 14 |
| Lenoir | 13 |
| Hickory | 7 |
| WAN1 | 2 |
| WAN2 | 2 |
| WAN3 | 2 |
Table 3.2 Number of IP Addresses Needed
After adding the WAN links and router addresses, we can say that we need 7 subnets, with anywhere from 2 to 25 IP addresses in each.
What about Growth?
Data networks seem to have a life of their own. It is a rare network that does not change and grow. As your users become comfortable with the applications they use via the network, they will start to ask for more features. You will probably find that you will be adding users, applications, servers and internetworking devices throughout the life of your network.
When you design an addressing plan, make sure you allow enough room for growth both in the number of subnets required and the number of addresses required in each subnet. The amount of growth depends almost entirely on your organization. What kind of expansion plans does your organization have? Are you more likely to add users/servers, or new branch offices? Are there any mergers/acquisitions anticipated for your future?
Choose the Proper Mask
The next step in creating your addressing plan is choosing a mask to be used in your network.
Going back to how a mask works, remember that each bit in the mask determines how the corresponding bit in the IP address is interpreted. Where there is a zero-bit in the mask, the corresponding bit of the IP address is part of the interface (host) identifier. Where there is a one-bit in the mask, the corresponding bit of the IP address is part of the network or subnet identifier.
So, the number of zero-bits in the mask determines the number of bits in the host field of an IP address, and thus the number of possible IP addresses for each subnet. Remember the formula 2 n 2 (where n is the number of bits)? Working backwards , you can determine the number of host bits required in the IP address given the number of addresses needed. The idea is to find the smallest value for n where the formula 2 n 2 gives you the number of addresses needed.
For example, if you need 25 addresses in a subnet, there must be at least five host bits in the IP address. That is, there must be at least five zeros in the mask: 2 4 2 = 14 (not enough); 2 5 2 = 30 (enough). If you need 1500 addresses, there must be at least 11 zeros in the mask (2 11 2 = 2046).
Consult the Tables
If youve been given a classfull block of addresses to usethat is, an entire class A, B, or C network addressthen you can refer to the corresponding subnet tables at the end of the chapter. Those tables can guide you to the proper mask to choose and how to allocate address ranges.
Lets look at our sample network shown in Figure 3.1. After our analysis, Table 3.2 showed that we need to support seven subnets, and the maximum number of addresses needed in any subnet is 25. Lets assume weve been given class C network 192.168.153.0 to use in our organization.
Table 3.3 is a traditional (RFC 950) Class C subnetting table. Consulting this table, we can try to find an appropriate mask.
| # Subnet Bits | # Subnets | # Host Bits | # Hosts | Mask |
| 2 | 2 | 6 | 62 | 255.255.255.192 |
| 3 | 6 | 5 | 30 | 255.255.255.224 |
| 4 | 14 | 4 | 14 | 255.255.255.240 |
| 5 | 30 | 3 | 6 | 255.255.255.248 |
| 6 | 62 | 2 | 2 | 255.255.255.252 |
Table 3.3 Class C Subnet Table
Can you locate a mask that will support seven subnets with 25 hosts each? No; a mask of 255.255.255.224 gives us enough host addresses, but not enough subnets, and 255.255.255.240 supports enough subnets, but not enough host addresses. Now what? In this situation, you have four options:
1. Use unnumbered interfaces.
2. Ask for a bigger block of addresses.
3. Play some tricks with your router.
4. Use subnet zero.
Use Unnumbered Interfaces
Many popular routers today provide a feature known as unnumbered interfaces or IP unnumbered . This feature can be used when the interface connects to a point-to-point network, such as a leased 56k or T1 line. When you use this feature, the point-to-point network does not need IP addresses and can be omitted from the total number of subnets. If we took advantage of this feature in our sample network, we would need to provide addresses only for the LAN segments. This can lead to substantial savings in the number of IP addresses needed. Well look at some examples in the next section.
One disadvantage of using unnumbered interfaces is that you cannot directly access those interfaces for testing or management purposes. So you will have to make a choice for manageability or for address conservation. In most networks, the choice will be clear, based on the needs of the organization. In other networks, you may just have to make a judgement call.
Using unnumbered interfaces in our example eliminates the need for three subnetsthe three WAN connections. Now we need only four subnets, and a mask of 255.255.255.224 would be appropriate.
Ask for a Bigger Block of Addresses
If you had two class C addresses, you could use one for the Headquarters LAN, and subnet the other for the branch LANs and WAN links. For example, if you were allocated two class C addresses (192.168.8.0 and 192.168.9.0), you could use 192.168.8.0 with the mask 255.255.255.0 for the Headquarters LAN. For the remaining LANs and WAN links we can subnet 192.168.9.0 with the mask 255.255.255.224. This gives us six subnets with 30 host addresses eachplenty to cover our needs.
Router Tricks
Most routers allow you to assign more than one IP address to an interface. This feature is called multinetting or secondary interfaces . Thus, you can actually support more than one subnet on a single router interface. In our sample network, you could use the mask 255.255.255.240 (which gives you 14 subnets and 14 host addresses), then assign two addresses on the Headquarters LAN interface of the router.
Caution
The choice of the two addresses is important. The first address must be a valid address on one subnet, the second address must be a valid address on another subnet.
Now we have 28 addresses available on the Headquarters LAN. Pretty handy, right? Yes, but at a price.
Remember that the Internet Protocol (IP) determines local vs remote delivery using the IP address. If your workstation is communicating with a host on another subnet (as determined by your mask and the target IP address), the datagrams will be delivered to your default gateway (router). Take a look at Figure 3.2.
Figure 3.2 Multiple subnets on a LAN segment.
WS1 is on one IP network, and WS2 and the server are on another. They (and the router) are all on a single LAN segment (i.e., they are all connected to the same Ethernet hub).
When WS2 wants to communicate with the server, the IP software in WS2 determines that, based on the mask of 255.255.255.0, the server is on the same IP network/subnet. So, WS2 will send a packet directly to the server.
What happens when WS1 wants to talk to the server? Are they on the same IP network? They arent, so WS1 will send the packets to its default gateway (Router1). Router1 will then forward the packets to the proper network for the server. Thus, each packet transmitted between WS1 and the server will appear on the Ethernet segment twice once from WS1 to the router and again from the router to the server (and vice-versa).
Tip
If you choose to use this trick, you need to be careful about which devices you place in which network/subnet. Try to keep devices that talk to each other on the same subnet.
Use Subnet Zero
Note
In the original subnetting standard (RFC 950), the subnets whose binary subnet ID is all zeros or all ones could not be used (thus the 2 in the subnetting formula 2 n 2). In RFC 1812, this restriction has been lifted. Here is a quote from RFC 1812:
Previous versions of this document also noted that subnet numbers must be neither 0 nor 1, and must be at least two bits in length. In a CIDR world, the subnet number is clearly an extension of the network prefix and cannot be interpreted without the remainder of the prefix. This restriction of subnet numbers is therefore meaningless in view of CIDR and may be safely ignored.
To help avoid potential interoperability problems, conservative network managers still follow the original specification and choose not to use the all zeros and all ones subnets. If this is the path you choose to follow, then you must subtract two from the number of subnets shown in each row of the tables at the end of the chapter. In some cases, such as the example were working on, it may be necessary to go ahead and use the additional subnets.
In our example, you could choose to use 255.255.255.224 as your mask, which gives you enough host addresses. By using subnet zero, you would have enough subnets to cover your needs.
For more practice choosing the correct mask for your network, please refer to the exercises at the end of the chapter.
Obtain IP Addresses
If you have already been given a block of addresses to use, and that block is sufficient for your needs, you may proceed to the next step (calculating the appropriate address ranges for each subnet).
If you have not been given any addresses, or if you determine that the addresses youve been given are not sufficient, then you will need to obtain one or more blocks of addresses. You should try these three sources in order:
1. Your organizations network manager
2. Your Internet Service Provider
3. The Internet Address Registry
Warning
Keep in mind one important reality: IP Addresses are a scarce and valuable commodity. No matter who supplies your addresses, they are under pressure to allocate addresses efficiently . It is likely that you will be asked to justify your request. Most often, your request will be honored as long as you can document that you will actually use at least half the addresses you ask for in the near future.
From Your Organizations Network Manager
In most organizations of any size at all, there is, or at least there should be , one person (or a small group ) responsible for allocating IP addresses to individuals and groups. Your first source of IP addresses would be such a resource.
From Your ISP
If your organization does not have a central allocation resource, or if you are that resource, then you may have to go outside your organization to obtain the addresses.
If you plan to connect to the Internet, then you must use either globally unique addresses, or private addresses and network address translation (refer to Chapters 4 and 5). If you do not plan to connect to the Internet (really ? ), then technically, you can use any addresses you want. However, RFC 1918 recommends that you use the addresses set aside for such purposes. Again, refer to Chapter 4 for details.
To obtain globally unique addresses, you should contact your Internet service provider (ISP) and present your request. You will be allocated a block of addresses that is a subset of the block that your ISP has been assigned.
For IT Professionals Only
Hierarchical Allocation of IP Addresses
This hierarchical allocation of addressesfrom Internet registry, to major ISP to minor ISP, to end user increases the efficiency of the overall Internet by keeping major block of IP addresses together, thus reducing the size of the core routing tables.
From Your Internet Registry
The ultimate source for IP addresses is the Internet Registry that has jurisdiction in your country. There are currently three regional registries:
ARIN: American Registry of Internet Numbers (www.arin.net). ARIN has jurisdiction for North America, South America, sub-Saharan Africa, and the Caribbean.
RIPE NCC (www.ripe.net). European Registry.
APNIC (www.apnic.net). Asia Pacific Registry.
Note
Addresses obtained directly from an Internet Registry such as ARIN are guaranteed to be unique, but they are not guaranteed to be globally routable in fact, you can almost count on the fact that they wont be. To make them work on the global Internet, you will need to be a peer on the Internet, interconnecting with other major ISPs.
RFC 2050 describes in more detail the policies regarding IP address allocation.
Calculate Ranges of IP Addresses for Each Subnet
Lets recap. So far we have
Determined our addressing requirements
Chosen the proper mask
Obtained sufficient IP addresses
Now its time to determine the appropriate range of addresses for each subnet.
Doing It the Hard Way
If you find yourself without any tools, you can always fall back to the manual method. There are shortcuts floating around on the grapevine that work in certain circumstances, but not in others. The following procedure works with all classes of addresses and all masks. Lets apply the procedure to our sample network.
First, identify the number of locally administered bits in your network address. In our example, weve been assigned a class C network (192.168.153.0). Class C networks have 24 network bits and 8 local bits.
Second, make a place for each of the local bitseight of them in our example:
_ _ _ _ _ _ _ _
Next, using the mask, we designate the subnet bits and the host bits. In our example, we chose 255.255.255.224 as our mask. Consulting Table 3.3, we see that this mask specifies three subnet bits and five host bits.
Subnet | Host
_ _ _ | _ _ _ _ _
Now we can start plugging in various combinations of valid bit patterns as we learned in Chapter 2. Three bits can be combined in 2 3 (8) combinations as listed:
000 100
001 101
010 110
011 111
In our example, we chose to use subnet zero, so well start there. Filling in the valid subnet bits into our template, we have
Subnet | Host
0 0 0 | X X X X X
Remember, for each subnet there are four meaningful addresses:
The subnet address (host bits all zero)
The first assignable IP address
The last assignable IP address
The broadcast address (host bits all ones)
So our first subnet looks like this:
Subnet | Host
0 0 0 | 0 0 0 0 0 = 0 (subnet address)
0 0 0 | 0 0 0 0 1 = 1 (subnet + 1)
|
0 0 0 | 1 1 1 1 0 = 30 (broadcast 1)
0 0 0 | 1 1 1 1 1 = 31 (broadcast address)
The first subnet address is 192.168.153.0, the range of addresses assignable to various devices is 192.168.153.1 through 192.168.153.30, and the broadcast address for the subnet is 192.168.153.31.
If we repeat the process for the other subnets, we simply use a different subnet bit pattern for each. The second subnet would be calculated as follows :
Subnet | Host
0 0 1 | 0 0 0 0 0 = 32 (subnet address)
0 0 1 | 0 0 0 0 1 = 33 (subnet + 1)
|
0 0 1 | 1 1 1 1 0 = 62 (broadcast 1)
0 0 1 | 1 1 1 1 1 = 63 (broadcast address)
Continuing through all eight possible subnets, we can summarize in Table 3.4.
| Subnet Address | First Assignable | Last Assignable | Broadcast Address | |||||
| 192.168.153.0 | 192.168.153.1 | 192.168.153.30 | 192.168.153.31 | |||||
| 192.168.153.32 | 192.168.153.33 | 192.168.153.62 | 192.168.153.63 | |||||
| 192.168.153.64 | 192.168.153.65 | 192.168.153.94 | 192.168.153.95 | |||||
| 192.168.153.96 | 192.168.153.97 | 192.168.153.126 | 192.168.153.127 | |||||
| 192.168.153.128 | 192.168.153.129 | 192.168.153.158 | 192.168.153.159 | |||||
| 192.168.153.160 | 192.168.153.161 | 192.168.153.190 | 192.168.153.191 | |||||
| 192.168.153.192 | 192.168.153.193 | 192.168.153.222 | 192.168.153.223 | |||||
| 192.168.153.224 | 192.168.153.225 | 192.168.153.254 | 192.168.153.255 | |||||
Table 3.4 Summary of Addresses for the Example Network
Table 3.4, along with all other possibilities for any network/mask combination, can also be found at the end of this chapter.
Worksheets
Doing it the hard way can be intellectually satisfying . However, when you want to get real work done, some simple tools can often save you a lot of time. For example, a series of tabular worksheets can serve the dual purpose of helping you calculate address ranges and tracking the assignment of addresses to devices on your network. Table 3.5 is the beginning few rows of a subnet assignment worksheet. The full worksheet (with addresses from zero to 255) is located at the end of the chapter.
| Addr | .128 | .192 | .224 | .240 | .248 | .252 | Assigned To |
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Table 3.5 Subnet Assignment Worksheet
The worksheet provides a visual reference to the addresses that are valid for each subnet, regardless of the mask used. For example, if we had chosen a mask of 255.255.255.248, the range of addresses available in the first subnet would be 192.168.153.1 through 192.168.153.6. The second subnet would contain 192.168.153.9 through 192.168.153.14. This is the same result that we would have obtained by doing the calculations the hard way or by using the subnetting tables.
The second benefit of a worksheet like this is that it is self-documenting . As you assign subnets, you can write in the column (under the appropriate mask) descriptive information about the subnetwhere it is located, technical contact, etc. You can also track individual address assignments by filling in information in the Assigned To column.
The worksheet is also scaleable . Each worksheet can document a single class C network. If you have to track allocations for a class B network, you can use one worksheet to document each group of 256 addresses, then one more worksheet to show a summary of the groups.
Subnet Calculators
Probably the easiest way to calculate address ranges is to use a subnet calculator. There are many such calculators available on the Internet as freeware or shareware. (See the FAQs for sources.) Using the IP Subnet Calculator from Net3 Group (www.net3group.com), we can calculate the address ranges for the subnets in our sample network.
First, we tell the calculator that we are using network 192.168.153.0 (a class C address), and a mask of 255.255.255.224 as shown in Figure 3.3.
Figure 3.3 IP Subnet Calculator.
Then, we simply click on the Subnets/Hosts tab to reveal the usable address ranges as shown in Figure 3.4.
Figure 3.4 Assignable address ranges.
Again, the results seen here match those obtained manually and from worksheets. By clicking the button above the CIDR tab, the calculator will copy the table shown to the Windows clipboard. You can then paste the table into a spreadsheet or other tools for further manipulation.
Allocate Addresses to Devices
Weve finally arrived at the goal of the exerciseto allocate individual addresses to the IP devices in our network.
Assigning Subnets
The first step is to assign subnets to appropriate network segments. Revisiting our network segments (from Table 3.2) we can now add a third column for the subnets assigned to each segment, as shown in Table 3.6.
| Subnet | IP Addresses | Subnet(s) |
| Headquarters | 25 | 192.168.153.32 |
| Morganton | 14 | 192.168.153.64 |
| Lenoir | 13 | 192.168.153.96 |
| Hickory | 7 | 192.168.153.128 |
| WAN1 | 2 | 192.168.153.160 |
| WAN2 | 2 | 192.168.153.192 |
| WAN3 | 2 | 192.168.153.0 |
Table 3.6 Subnet Assignment
Is this the only way to assign the subnets? Absolutely not: Pick any of the eight subnets and assign them to any of the seven network segments. Technically, it makes no difference at all which subnet is assigned to which segment. The only factor to consider here is ease of use and documentation.
Notice that subnet zero was allocated to one of the WAN links. Since we cant be totally conservative herewe must use subnet zero, well allocate it to a network segment that is least likely to have interoperability problems. The idea here is that most routers purchased in the last few years do support the subnet zero feature without any problems.
Assigning Device Addresses
Once youve assigned subnets to the various network segments, its time to assign individual addresses to devices that need them. Here again is where the worksheets come in handy. Lets assign addresses for the Hickory subnet in our sample network. Table 3.7 contains another excerpt from the address assignment worksheet.
| Addr | .128 | .192 | .224 | .240 | .248 | .252 | Assigned To |
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Table 3.7 Subnet Assignment WorksheetHickory
Again, there is no one correct way to do these assignmentsits up to you. There are basically three schools of thought on the matter: sequential allocation, reserved addresses, and grow towards the middle.
Sequential Allocation
In Table 3.7, we simply assigned the next available IP address to each device without too much regard to the type or function of the device. The advantages to this approach are flexibility, and no wasted addresses. The disadvantages include no order or scheme of assignment, and no way to determine the function of the device based on its address.
Reserved Addresses
The second approach consists of reserving a range of addresses in each subnet for various functions. For example,
Routers: first three addresses
Servers: next five addresses
Misc: next five addresses (printer, smart hubs, etc.)
Workstations: all remaining addresses
The advantage here is that you (and your support staff) can readily determine the kind of device based on its address. Conversely, given a device, you can determine its address. The main disadvantage is that the reserved addresses can go unused, while there may be a need for more addresses in other functional groups.
Grow Towards the Middle
The third technique is to assign the main subnet router the first available address on the subnet, then assign the next higher addresses in sequence to other internetworking and support devices. Workstations are assigned addresses from the top of the address range down, as needed.
This technique allows all available addresses to be used, while preserving some kind of functional consistency.
Use the technique with which you are most comfortable. Many administrators use a combination of the three techniques.
Document Your Work
Congratulations! Youve completed the assignment of IP addresses to all the networked devices that need them. Time to relaxalmost.
Keeping Track of What Youve Done
Youve spent quite a bit of time so far working out the details of this project. A small additional investment of time can yield big dividends down the road. Yes, were talking about documentationagain.
If youve used the worksheet method of allocation addresses, then your work is done. If you used an IP calculator or the back of a napkin, you should probably transfer your work to something more permanent.
Paper
At the very least, write down what you have done:
Address blocks obtained
Mask chosen
Subnets assigned
IP addresses assigned (and to whom)
Keep your notes where they can be updated when things change.
Spreadsheets
With a little work, you can create a significant source of information by putting your assignment data into a spreadsheet. Create columns for:
IP address
Date assigned
Assigned to
Contact information (phone, fax, e-mail)
Device type
Many spreadsheet applications provide for a simple data entry form to assist in loading the information. Throughout the life of your network you can query, sort , and report on information in the spreadsheet to give you assignments by name, by address, by type, by date, and so on. When the time comes for an upgrade, wouldnt it be nice to have a way to identify quickly the addresses and locations of all your routers?
Databases
Just about anything you can do with a spreadsheet, you can do with a database application as well. Most database software will allow you to create input forms with data validation to help keep errors out, and most provide report-writing capability to produce standard and ad hoc reports .
The IP address allocation database does not have to be very sophisticated to be effective. A simple one-table database in Microsoft Access, for example, can provide appropriate information for a very large organization.
Many new Network Management applications now on the market provide asset management functions where networked devices are tracked. Use these facilities to record allocation and contact information as listed earlier.
In Any Case
No network is static. Users come and go; applications just seem to keep coming. Technology changes. Many network designers are replacing routers with layer-2 and layer-3 switches. Keep your documentation up to date! Out of date information is, in some ways, worse than no information at all.
Summary
In this chapter, we have presented the steps required to develop an effective IP addressing plan for networks with fixed masks. First, we determined the number of IP addresses and subnets actually needed, with some hints for squeezing the most out of the addresses youve been given. Using subnetting tables, we determined the proper mask to use. Next came the calculation of appropriate address ranges using manual techniques, worksheets, or subnet calculators. We then assigned IP addresses to those devices that needed them. Finally, we discussed the importance of properly documenting our work.
FAQs
Q: What if my ISP or network administrator wont give me the number of addresses I need?
A: First, be sure your request is realistic. If youre paying $10 per month for dial-up Internet access, dont be surprised if your ISP wont give you 16 class C network prefixes! In fact, if you are simply subscribing to monthly dial-up access, your ISP probably wont want to give you any permanent IP addresses. If, however, you are purchasing a T1 line for full-time access, your ISP probably will be more likely to be generous with a block of addresses. In general, if you can document that you will be implementing at least half of the addresses requested within six months, your request should be granted. If your ISP is still reluctant to give you enough addresses, you may have to rely on other techniques, such as variable-length subnetting (Chapter 6) or private addresses (Chapter 4).
Q: Where can I get a subnet calculator?
A: URLs:
http://www.net3group.com/download.asp
Downloadable stand-alone application that runs under Win 95/98/NT.
http://www.cisco.com/techtools/ip_addr.html
Online calculator.
http://www.ccci.com/subcalc/download.htm
Java-based calculator.
http://www.ajw.com/ipcalc.htm
Calculator for the Palm Pilot.
Exercises
1. Youve been assigned a /23 CIDR block. How many traditional Class C networks does that represent? What is the equivalent net mask? How many total host addresses does the block cover?
Answer: 2 class Cs; 255.255.254.0; 512 addresses
2. What mask would you use if you needed to divide a class B network into 200 subnets with 100 addresses needed in each?
Answer: There are two possible masks: 255.255.255.0 and 255.255.255.128. Since we were not given any information about growth, we need to pick the one most likely to meet our future needs. The most common choice would probably be 255.255.255.0 since it is easy to use and allows some growth in the number of subnets and significant growth in the size of each subnet.
3. Two routers are connected via a leased T1 line. Do these router interfaces need an IP address? Why or why not?
Answer: In general, the answer is yes. However, if the routers support the IP unnumbered feature, they do not.
4. Under what circumstances would you use a fixed-length subnetting scheme?
Answer: You must use a fixed-length subnetting scheme if you are using a routing protocol that does not support variable-length subnetting. Of the common IP routing protocols in use today, RIP (v. 1) and Ciscos IGRP require fixed-length subnetting. RIP2, EIGRP, and OSPF support variable-length subnetting. When using those protocols, you still may want to choose fixed-length subnetting for simplicity.
5. Using any method you prefer, calculate the address ranges for all the subnets created in a Class B network using the mask 255.255.254.0. Use the all-zeros and all-ones subnets.
Answer: 128 subnets as follows:
N.N.0.0 N.N.1.255
N.N.2.0 N.N.3.255
N.N.4.0 N.N.5.255
N.N.254.0 N.N.255.255
6. What size CIDR block would you ask for if you needed 420 subnets with 170 host addresses each?
Answer: Based on the 170-address requirement, you would choose a mask of 255.255.255.0. In other words, you need eight bits to cover the host addresses. You need another nine bits to cover the number of subnets for a total need of 17 bits. Since an IP address is 32 bits long, and you need 17 for you own use, you would ask for a (32 17) or 15-bit block (/15 in CIDR notation).
7. Why cant you use a mask of 255.255.255.254?
Answer: The host field needs to be at least two bits long. A host field of all zeros denotes the subnet address, and a host field of all ones is the broadcast address for that subnet.
8. Why should you bother documenting your address assignments?
Answer: To help with future assignments, to assist with troubleshooting activities, to help with upgrades, to prevent duplicate address assignments.
Subnetting Tables
Note that these tables comply with RFC.
Class A Subnetting Table
| # Subnet Bits | # Subnets | # Host Bits | # Hosts | Mask |
| 1 | 2 | 23 | 8,388,608 | 255.128.0.0 |
| 2 | 4 | 22 | 4,194,302 | 255.192.0.0 |
| 3 | 8 | 21 | 2,097,150 | 255.224.0.0 |
| 4 | 16 | 20 | 1,048,574 | 255.240.0.0 |
| 5 | 32 | 19 | 524,286 | 255.248.0.0 |
| 6 | 64 | 18 | 262,142 | 255.252.0.0 |
| 7 | 128 | 17 | 131,070 | 255.254.0.0 |
| 8 | 256 | 16 | 65,534 | 255.255.0.0 |
| 9 | 512 | 15 | 32,766 | 255.255.128.0 |
| 10 | 1,024 | 14 | 16,382 | 255.255.192.0 |
| 11 | 2,048 | 13 | 8,190 | 255.255.224.0 |
| 12 | 4,096 | 12 | 4,094 | 255.255.240.0 |
| 13 | 8,192 | 11 | 2,046 | 255.255.248.0 |
| 14 | 16,384 | 10 | 1,022 | 255.255.252.0 |
| 15 | 32,768 | 9 | 510 | 255.255.254.0 |
| 16 | 65,536 | 8 | 254 | 255.255.255.0 |
| 17 | 131,072 | 7 | 126 | 255.255.255.128 |
| 18 | 262,144 | 6 | 62 | 255.255.255.192 |
| 19 | 524,288 | 5 | 30 | 255.255.255.224 |
| 20 | 1,048,576 | 4 | 14 | 255.255.255.240 |
| 21 | 2,097,152 | 3 | 6 | 255.255.255.248 |
| 22 | 4,194,304 | 2 | 2 | 255.255.255.252 |
Subnet First Host Last Host Subnet Broadcast
1 Bit (255.128.0.0)
N.0.0.0 N.0.0.1 N.127.255.254 N.127.255.255
N.128.0.0 N.128.0.1 N.255.255.254 N.255.255.255
2 Bits (255.192.0.0)
N.0.0.0 N.0.0.1 N.63.255.254 N.63.255.255
N.64.0.0 N.64.0.1 N.127.255.254 N.127.255.255
N.128.0.0 N.128.0.1 N.191.255.254 N.191.255.255
N.192.0.0 N.192.0.1 N.255.255.254 N.255.255.255
3 Bits (255.224.0.0
N.0.0.0 N.0.0.1 N.31.255.254 N.31.255.255
N.32.0.0 N.32.0.1 N.63.255.254 N.63.255.255
. . .
N.192.0.0 N.192.0.1 N.223.255.254 N.223.255.255
N.224.0.0 N.224.0.1 N.255.255.254 N.255.255.255
4 Bits (255.240.0.0)
N.0.0.0 N.0.0.1 N.15.255.254 N.15.255.255
N.16.0.0 N.16.0.1 N.31.255.254 N.31.255.255
. . .
N.224.0.0 N.224.0.1 N.239.255.254 N.239.255.255
N.240.0.0 N.240.0.1 N.255.255.254 N.255.255.255
5 Bits (255.248.0.0)
N.0.0.0 N.0.0.1 N.7.255.254 N.7.255.255
N.8.0.0 N.8.0.1 N.15.255.254 N.15.255.255
. . .
N.240.0.0 N.240.0.1 N.247.255.254 N.247. 255.255
N.248.0.0 N.248.0.1 N.255.255.254 N.255.255.255
6 Bits (255.252.0.0)
N.0.0.0 N.0.0.1 N.3.255.254 N.3.255.255
N.4.0.0 N.4.0.1 N.7.255.254 N.7.255.255
. . .
N.248.0.0 N.248.0.1 N.251.255.254 N.251. 255.255
N.252.0.0 N.252.0.1 N.255.255.254 N.255.255.255
7 Bits (255.254.0.0)
N.0.0.0 N.0.0.1 N.1.255.254 N.1.255.255
N.2.0.0 N.2.0.1 N.3.255.254 N.3.255.255
. . .
N.252.0.0 N.252.0.1 N.253.255.254 N.253.255.255
N.254.0.0 N.254.0.1 N.255.255.254 N.255.255.255
8 Bits (255.255.0.0)
N.0.0.0 N.0.0.1 N.0.255.254 N.0.255.255
N.1.0.0 N.1.0.1 N.1.255.254 N.1.255.255
. . .
N.254.0.0 N.254.0.1 N.254.255.254 N.254.255.255
N.255.0.0 N.255.0.1 N.255.255.254 N.255.255.255
9 Bits (255.255.128.0)
N.0.0.0 N.0.0.1 N.0.127.254 N.0.127.255
N.0.128.0 N.0.128.1 N.0.255.254 N.0.255.255
N.1.0.0 N.1.0.1 N.1.127.254 N.1.127.255
N.1.128.0 N.1.128.1 N.1.255.254 N.1.255.255
. . .
N.255.0.0 N.255.0.1 N.255.127.254 N.255.127.255
N.255.128.0 N.255.128.1 N.255.255.254 N.255.255.255
10 Bits (255.255.192.0)
N.0.0.0 N.0.0.1 N.0.63.254 N.0.63.255
N.0.64.0 N.0.64.1 N.0.127.254 N.0.127.255
N.0.128.0 N.0.128.1 N.0.191.254 N.0.191.255
N.0.192.0 N.0.192.1 N.0.255.254 N.0.255.255
N.1.0.0 N.1.0.1 N.1.63.254 N.1.63.255
N.1.64.0 N.1.64.1 N.1.127.254 N.1.127.255
. . .
N.255.128.0 N.255.128.1 N.255.191.254 N.255.191.255
N.255.192.0 N.255.192.1 N.255.255.254 N.255.255.255
11 Bits (255.255.224.0)
N.0.0.0 N.0.0.1 N.0.31.254 N.0.31.255
N.0.32.0 N.0.32.1 N.0.63.254 N.0.63.255
N.0.64.0 N.0.64.1 N.0.127.254 N.0.127.255
. . .
N.255.192.0 N.255.192.1 N.255.223.254 N.255.223.255
N.255.224.0 N.255.224.1 N.255.255.254 N.255.255.255
12 Bits (255.255.240.0)
N.0.0.0 N.0.0.1 N.0.15.254 N.0.15.255
N.0.16.0 N.0.16.1 N.0.31.254 N.0.31.255
N.0.32.0 N.0.32.1 N.0.47.254 N.0.47.255
. . .
N.255.224.0 N.255.224.1 N.255.239.254 N.255.239.255
N.255.240.0 N.255.240.1 N.255.255.254 N.255.255.255
13 Bits (255.255.248.0)
N.0.0.0 N.0.0.1 N.0.7.254 N.0.7.255
N.0.8.0 N.0.8.1 N.0.15.254 N.0.15.255
N.0.16.0 N.0.16.1 N.0.23.254 N.0.23.255
. . .
N.255.240.0 N.255.240.1 N.255.247.254 N.255.247.255
N.255.248.0 N.255.248.1 N.255.255.254 N.255.255.255
14 Bits (255.255.252.0)
N.0.0.0 N.0.0.1 N.0.3.254 N.0.3.255
N.0.4.0 N.0.4.1 N.0.7.254 N.0.7.255
N.0.8.0 N.0.8.1 N.0.11.254 N.0.11.255
. . .
N.255.248.0 N.255.248.1 N.255.251.254 N.255.251.255
N.255.252.0 N.255.252.1 N.255.255.254 N.255.255.255
15 Bits (255.255.254.0)
N.0.0.0 N.0.0.1 N.0.1.254 N.0.1.255
N.0.2.0 N.0.2.1 N.0.3.254 N.0.3.255
N.0.4.0 N.0.4.1 N.0.5.254 N.0.5.255
. . .
N.255.252.0 N.255.252.1 N.255.253.254 N.255.253.255
N.255.254.0 N.255.254.1 N.255.255.254 N.255.255.255
16 Bits (255.255.255.0)
N.0.0.0 N.0.0.1 N.0.0.254 N.0.0.255
N.0.1.0 N.0.1.1 N.0.1.254 N.0.1.255
N.0.2.0 N.0.2.1 N.0.2.254 N.0.2.255
. . .
N.255.254.0 N.255.254.1 N.255.254.254 N.255.254.255
N.255.255.0 N.255.255.1 N.255.255.254 N.255.255.255
17 Bits (255.255.255.128)
N.0.0.0 N.0.0.1 N.0.0.126 N.0.0.127
N.0.0.128 N.0.0.129 N.0.0.254 N.0.0.255
N.0.1.0 N.0.1.1 N.0.1.126 N.0.1.127
N.0.1.128 N.0.1.129 N.0.1.254 N.0.1.255
. . .
N.255.255.0 N.255.255.1 N.255.255.126 N.255.255.127
N.255.255.128 N.255.255.129 N.255.255.254 N.255.255.255
18 Bits (255.255.255.192)
N.0.0.0 N.0.0.1 N.0.0.62 N.0.0.63
N.0.0.64 N.0.0.65 N.0.0.126 N.0.0.127
N.0.0.128 N.0.0.129 N.0.0.190 N.0.1.191
N.0.0.192 N.0.0.193 N.0.0.254 N.0.1.255
N.0.1.0 N.0.1.1 N.0.1.62 N.0.1.63
. . .
N.255.255.128 N.255.255.129 N.255.255.190 N.255.255.191
N.255.255.192 N.255.255.193 N.255.255.254 N.255.255.255
19 Bits (255.255.255.224)
N.0.0.0 N.0.0.1 N.0.0.30 N.0.0.31
N.0.0.32 N.0.0.33 N.0.0.62 N.0.0.63
N.0.0.64 N.0.0.65 N.0.0.94 N.0.0.95
N.0.0.96 N.0.0.97 N.0.0.126 N.0.0.127
N.255.255.192 N.255.255.193 N.255.255.222 N.255.255.223
N.255.255.224 N.255.255.225 N.255.255.254 N.255.255.255
20 Bits (255.255.255.240)
N.0.0.0 N.0.0.1 N.0.0.14 N.0.0.15
N.0.0.16 N.0.0.16 N.0.0.30 N.0.0.31
N.0.0.32 N.0.0.33 N.0.0.46 N.0.0.47
N.255.255.224 N.255.255.225 N.255.255.238 N.255.255.239
N.255.255.240 N.255.255.241 N.255.255.254 N.255.255.255
21 Bits (255.255.255.248)
N.0.0.0 N.0.0.1 N.0.0.6 N.0.0.7
N.0.0.8 N.0.0.9 N.0.0.14 N.0.0.15
N.0.0.16 N.0.0.17 N.0.0.22 N.0.0.23
N.255.255.240 N.255.255.241 N.255.255.246 N.255.255.247
N.255.255.248 N.255.255.249 N.255.255.254 N.255.255.255
22 Bits (255.255.255.252)
N.0.0.0 N.0.0.1 N.0.0.2 N.0.0.3
N.0.0.4 N.0.0.5 N.0.0.6 N.0.0.7
N.0.0.8 N.0.0.9 N.0.0.10 N.0.0.11
N.255.255.248 N.255.255.249 N.255.255.250 N.255.255.251
N.255.255.252 N.255.255.253 N.255.255.254 N.255.255.255
Class B Subnetting Table
| # Subnet Bits | # Subnets | # Host Bits | # Hosts | Mask |
| 1 | 2 | 15 | 32,766 | 255.255.128.0 |
| 2 | 4 | 14 | 16,382 | 255.255.192.0 |
| 3 | 8 | 13 | 8,190 | 255.255.224.0 |
| 4 | 16 | 12 | 4,094 | 255.255.240.0 |
| 5 | 32 | 11 | 2,046 | 255.255.248.0 |
| 6 | 64 | 10 | 1,022 | 255.255.252.0 |
| 7 | 128 | 9 | 510 | 255.255.254.0 |
| 8 | 256 | 8 | 254 | 255.255.255.0 |
| 9 | 512 | 7 | 126 | 255.255.255.128 |
| 10 | 1,024 | 6 | 62 | 255.255.255.192 |
| 11 | 2,048 | 5 | 30 | 255.255.255.224 |
| 12 | 4,096 | 4 | 14 | 255.255.255.240 |
| 13 | 8,192 | 3 | 6 | 255.255.255.248 |
| 14 | 16,384 | 2 | 2 | 255.255.255.252 |
1 Bit (255.255.128.0)
N.N.0.0 N.N.0.1 N.N.127.254 N.N.127.255
N.N.128.0 N.N.128.1 N.N.191.254 N.N.191.255
2 Bits (255.255.192.0)
N.N.0.0 N.N.0.1 N.N.63.254 N.N.63.255
N.N.64.0 N.N.64.1 N.N.127.254 N.N.127.255
N.N.128.0 N.N.128.1 N.N.191.254 N.N.191.255
N.N.192.0 N.N.192.1 N.N.255.254 N.N.255.255
3 Bits (255.255.224.0)
N.N.0.0 N.N.0.1 N.N.31.254 N.N.31.255
N.N 32.0 N.N.32.1 N.N.63.254 N.N.63.255
N.N.64.0 N.N.64.1 N.N.95.254 N.N.95.255
. . .
N.N.192.0 N.N.192.1 N.N.223.254 N.N.223.255
N.N.224.0 N.N.224.1 N.N.255.254 N.N.255.255
4 Bits (255.255.240.0)
N.N.0.0 N.N.0.1 N.N.15.254 N.N.15.255
N.N 16.0 N.N.16.1 N.N.31.254 N.N.31.255
N.N.32.0 N.N.32.1 N.N.47.254 N.N.47.255
. . .
N.N.224.0 N.N.224.1 N.N.239.254 N.N.239.255
N.N.240.0 N.N.240.1 N.N.255.254 N.N.255.255
5 Bits (255.255.248.0)
N.N.0.0 N.N.0.1 N.N.7.254 N.N.7.255
N.N 8.0 N.N.8.1 N.N.15.254 N.N.15.255
N.N.16.0 N.N.16.1 N.N.23.254 N.N.23.255
. . .
N.N.240.0 N.N.240.1 N.N.247.254 N.N.247.255
N.N.248.0 N.N.248.1 N.N.255.254 N.N.255.255
6 Bits (255.255.252.0)
N.N.0.0 N.N.0.1 N.N.3.254 N.N.3.255
N.N 4.0 N.N.4.1 N.N.7.254 N.N.7.255
N.N.8.0 N.N.8.1 N.N.11.254 N.N.11.255
. . .
N.N.248.0 N.N.248.1 N.N.251.254 N.N.251.255
N.N.252.0 N.N.252.1 N.N.255.254 N.N.255.255
7 Bits (255.255.254.0)
N.N.0.0 N.N.0.1 N.N.1.254 N.N.1.255
N.N 2.0 N.N.2.1 N.N.3.254 N.N.3.255
N.N.4.0 N.N.4.1 N.N.5.254 N.N.5.255
. . .
N.N.252.0 N.N.252.1 N.N.253.254 N.N.253.255
N.N.254.0 N.N.254.1 N.N.255.254 N.N.255.255
8 Bits (255.255.255.0)
N.N.0.0 N.N.0.1 N.N.0.254 N.N.0.255
N.N 1.0 N.N.1.1 N.N.1.254 N.N.1.255
N.N.2.0 N.N.2.1 N.N.2.254 N.N.2.255
. . .
N.N.254.0 N.N.254.1 N.N.254.254 N.N.254.255
N.N.255.0 N.N.255.1 N.N.255.254 N.N.255.255
9 Bits (255.255.255.128)
N.N.0.0 N.N.0.1 N.N.0.126 N.N.0.127
N.N 0.128 N.N.0.129 N.N.0.254 N.N.0.255
N.N.1.0 N.N.1.1 N.N.1.126 N.N.1.127
N.N.1.128 N.N.1.129 N.N.1.254 N.N.1.255
. . .
N.N.255.0 N.N.255.1 N.N.255.126 N.N.255.127
N.N.255.128 N.N.255.129 N.N.255.254 N.N.255.255
10 Bits (255.255.255.192)
N.N.0.0 N.N.0.1 N.N.0.62 N.N.0.63
N.N 0.64 N.N.0.65 N.N.0.126 N.N.0.127
N.N 0.128 N.N.0.129 N.N.0.190 N.N.0.191
N.N.0.192 N.N.0.193 N.N.0.254 N.N.0.255
N.N.1.0 N.N.1.1 N.N.1.62 N.N.1.63
. . .
N.N.255.128 N.N.255.129 N.N.255.190 N.N.255.191
N.N.255.192 N.N.255.193 N.N.255.254 N.N.255.255
11 Bits (255.255.255.224)
N.N.0.0 N.N.0.1 N.N.0.30 N.N.0.31
N.N 0.32 N.N.0.33 N.N.0.62 N.N.0.63
N.N 0.64 N.N.0.65 N.N.0.94 N.N.0.95
. . .
N.N.255.192 N.N.255.192 N.N.255.222 N.N.255.223
N.N.255.224 N.N.255.225 N.N.255.254 N.N.255.255
12 Bits (255.255.255.240)
N.N.0.0 N.N.0.1 N.N.0.14 N.N.0.15
N.N 0.16 N.N.0.17 N.N.0.30 N.N.0.31
N.N 0.32 N.N.0.33 N.N.0.46 N.N.0.47
. . .
N.N.255.224 N.N.255.225 N.N.255.238 N.N.255.239
N.N.255.240 N.N.255.241 N.N.255.254 N.N.255.255
13 Bits (255.255.255.248)
N.N.0.0 N.N.0.1 N.N.0.6 N.N.0.7
N.N 0.8 N.N.0.9 N.N.0.14 N.N.0.15
N.N 0.16 N.N.0.17 N.N.0.22 N.N.0.23
. . .
N.N.255.240 N.N.255.241 N.N.255.246 N.N.255.247
N.N.255.248 N.N.255.249 N.N.255.254 N.N.255.255
14 Bits (255.255.255.252)
N.N.0.0 N.N.0.1 N.N.0.2 N.N.0.3
N.N 0.4 N.N.0.5 N.N.0.6 N.N.0.7
N.N 0.8 N.N.0.9 N.N.0.10 N.N.0.11
. . .
N.N.255.248 N.N.255.249 N.N.255.250 N.N.255.251
N.N.255.252 N.N.255.253 N.N.255.254 N.N.255.255
Class C Subnetting Table
| # Subnet Bits | # Subnets | # Host Bits | # Hosts | Mask |
| 1 | 2 | 7 | 126 | 255.255.255.128 |
| 2 | 4 | 6 | 62 | 255.255.255.192 |
| 3 | 8 | 5 | 30 | 255.255.255.224 |
| 4 | 16 | 4 | 14 | 255.255.255.240 |
| 5 | 32 | 3 | 6 | 255.255.255.248 |
| 6 | 64 | 2 | 2 | 255.255.255.252 |
Subnet First Host Last Host Subnet Broadcast
1 Bit (255.255.255.128)
N.N.N.0 N.N.N.1 N.N.N.126 N.N.N.127
N.N.N.128 N.N.N.129 N.N.N.254 N.N.N.255
2 Bits (255.255.255.192)
N.N.N.0 N.N.N.1 N.N.N.62 N.N.N.63
N.N.N.64 N.N.N.65 N.N.N.126 N.N.N.127
N.N.N.128 N.N.N.129 N.N.N.190 N.N.N.191
N.N.N.192 N.N.N.193 N.N.N.254 N.N.N.255
3 Bits (255.255.255.224)
N.N.N.0 N.N.N.1 N.N.N.30 N.N.N.31
N.N.N.32 N.N.N.33 N.N.N.62 N.N.N.63
N.N.N.64 N.N.N.65 N.N.N.94 N.N.N.95
N.N.N.96 N.N.N.97 N.N.N.126 N.N.N.127
N.N.N.128 N.N.N.129 N.N.N.158 N.N.N.159
N.N.N.160 N.N.N.161 N.N.N.190 N.N.N.191
N.N.N.192 N.N.N.193 N.N.N.222 N.N.N.223
N.N.N.224 N.N.N.225 N.N.N.254 N.N.N.255
4 Bits (255.255.255.240)
N.N.N.0 N.N.N.1 N.N.N.14 N.N.N.15
N.N.N.16 N.N.N.17 N.N.N.30 N.N.N.31
N.N.N.32 N.N.N.33 N.N.N.46 N.N.N.47
. . .
N.N.N.224 N.N.N.225 N.N.N.238 N.N.N.239
N.N.N.240 N.N.N.241 N.N.N.254 N.N.N.255
5 Bits (255.255.255.248)
N.N.N.0 N.N.N.1 N.N.N.6 N.N.N.7
N.N.N.8 N.N.N.9 N.N.N.14 N.N.N.15
N.N.N.16 N.N.N.17 N.N.N.22 N.N.N.23
. . .
N.N.N.240 N.N.N.241 N.N.N.246 N.N.N.247
N.N.N.248 N.N.N.249 N.N.N.254 N.N.N.255
6 Bits (255.255.255.252)
N.N.N.0 N.N.N.1 N.N.N.2 N.N.N.3
N.N.N.4 N.N.N.5 N.N.N.6 N.N.N.7
N.N.N.8 N.N.N.9 N.N.N.10 N.N.N.11
. . .
N.N.N.248 N.N.N.249 N.N.N.250 N.N.N.251
N.N.N.252 N.N.N.253 N.N.N.254 N.N.N.255
Subnet Assignment Worksheet
| Addr | .128 | .192 | .224 | .240 | .248 | .252 | Assigned To |
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EAN: N/A
Pages: 15