Forming Subnets from 24-Bit Network Addresses


Having explored the basics of subnetting, you are ready for a more in-depth examination of how they are formed and the mathematics on which they are founded. Starting with a 24-bit network address (equivalent to a Class C network), the formation of subnets is limited to between 2- and 6-bit masks. The limited size of masks in a 24-bit network makes for an ideal case study to demonstrate the potential for confusion as to where a subnet begins and ends and why some addresses look valid but aren't.

Let's start by examining the basic trade-off proposition that exists between the number of subnets that can be formed with any given mask size and the subsequent number of usable hosts per subnet. This information is presented Table 3-3.

Table 3-3. Hosts Versus Subnets in a 24-Bit Network

Number of Bits in the Network Prefix

Subnet Mask

Number of Usable Subnet Addresses

Number of Usable Hosts Per Subnet

2

255.255.255.192

2

62

3

255.255.255.224

6

30

4

255.255.255.240

14

14

5

255.255.255.248

30

6

6

255.255.255.252

62

2


NOTE

You might find other reference sources that report the data presented in Table 3-3 slightly differently. Many information sources apparently prefer to use "mathematically feasible" quantities as opposed to usable quantities. I define "usable" in the historical context of Classical IP and FLSM to exclude the mathematically feasible values of all 0s and all 1s. Thus, all my numbers are 2 less than other sources describing the mathematical potential of each subnet.


At first glance, the inverse correlation between subnets and hosts should be obvious. You can get either two subnets of 62 addresses each or 62 subnets of two addresses each. Another combination is six subnets of 30 hosts or 30 subnets with six hosts in each. Finally, a 4-bit subnet mask gives you exactly 14 subnets with 14 hosts. As I said, the inverse correlation between subnets and hosts in a 24-bit network should be obvious. What might not be so obvious are the inefficiencies inherent in each of the subnetting schemes or the areas of possible confusion. The potential inefficiencies of subnetting are explored later in this chapter.

Subnetting is confusing because you create artificial subdivisions within a network. This means that instead of host addresses within a "network" address starting predictably at 0 and ending at 255, initial host addresses can be almost any value in between. For the sake of understanding (and using) subnetwork addressing, you need to be able to identify three significant host addresses within each subnet:

  • Base address Each subnet begins with a host address of all 0s. This address is usually reserved for use in identifying the subnet itself and is not assigned to a specific host.

  • First assignable address The host address that is one up from the base is regarded as the first address that can be used for host identification.

  • Broadcast address Each subnet ends with the broadcast address. All hosts within the subnet accept incoming packets addressed to either themselves or the subnet's broadcast address. Thus, the broadcast address is a way of sending a single stream of IP packets to all the connected devices within a given subnet. In a properly designed addressing scheme, the broadcast address of any given subnet has a value of all 1s.

It is essential that you be able to map out these key addresses within a subnetted network, but the irony is that doing so won't really help you understand how subnets are formed. The logic behind subnetting is obvious only when you view those addresses in their binary form. Using just decimal numbers can become intuitive if you work with IP addresses long enough, but it is not intuitive at first, and it's certainly not the easy way to learn about subnetting.

In this section, you can see how flexible a tool subnetting can be, as well as how confusing. The role of certain addresses varies with the size of the mask. For example, it would be extremely useful to know the base address, first assignable address, and last address in a subnet. Unfortunately, these arbitrary designations absolutely require understanding how the subnets are formed. For the sake of example, we will take the address block of 192.169.125.0 and show you all the different ways it can be subnetted with fixed-length masks.

Subnetting with a 2-Bit Mask

A 2-bit mask identified via the subnet mask 255.255.255.192 yields a total of two usable subnet addresses, each containing 62 usable host addresses. There are 64 addresses in each subnet, but only 62 are usable due to the need to reserve the all-0s and all-1s addresses.

In the example presented in Table 3-4, the Class C address 192.169.125 is subnetted with a 2-bit mask. Notice how the base or starting address of each subnet is exactly 64 addresses higher than the preceding subnet. In other words, we are counting in multiples of 64. This is perfectly logical when you consider that the decimal equivalent value of the rightmost bit of the subnet field is 64. A hyphen is used to further highlight where the subnet address ends, and where the host field begins. You will not find "real" IP addresses formatted this way!

Table 3-4. Subnetting with a 2-Bit Mask in a 24-Bit Network
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101000.01111101.00000000

192.168.125.0

Subnet 0

11000000.10101000.01111101.00-000000

192.168.125.0

Subnet 1

11000000.10101000.01111101.01-000000

192.168.125.64

Subnet 2

11000000.10101000.01111101.10-000000

192.168.125.128

Subnet 3

11000000.10101000.01111101.11-000000

192.168.125.192


Mapping Significant Addresses

Subnetting, by virtue of logically segmenting a host address into two different fields, still must adhere to the precept of uniqueness. Host addresses, although they are duplicated across each newly created subnet, remain unique in the context of the extended network prefix. To better illustrate this point, consider the following example. You can have a value of 000001 in the 6-bit host address field in each of the subnets created. Host 000001 in subnet 2 (10 in binary) translates into 192.169.125.129. This is because the last octet, which is used for both host and subnet identification, is 10000001 in binary. Translated into decimal, the string is 129. Thus, the host addresses are not unique when viewed across subnets, but because each subnet has a unique 2-bit prefix, the entire octet remains unique. More importantly, this example demonstrates how unintuitive subnetting can be. Who would expect that host address 0.0.0.129 would be the first assignable number in a subnet?

As I mentioned earlier, subnetting really makes sense only when you look at the binary numbers. To better illustrate the logic behind this potentially confusing situation, Table 3-5 walks you through the process of mapping out the base address, first usable address, and broadcast address in 2-bit subnets.

Table 3-5. Mapping Important Addresses with a 2-Bit Mask
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.00-000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.00-000001

192.169.125.1

Subnet 0

Subnet 0

11000000.10101001.01111101.00-111111

192.169.125.63

Subnet 1

11000000.10101001.01111101.01-000000

192.169.125.64

Subnet 1

11000000.10101001.01111101.01-000001

192.169.125.65

Subnet 1

Subnet 1

11000000.10101001.01111101.01-111111

192.169.125.127

Subnet 2

11000000.10101001.01111101.10-000000

192.169.125.128

Subnet 2

11000000.10101001.01111101.10-000001

192.169.125.129

Subnet 2

Subnet 2

11000000.10101001.01111101.10-111111

192.169.125.191

Subnet 3

11000000.10101001.01111101.11-000000

192.169.125.192

Subnet 3

11000000.10101001.01111101.11-000001

192.169.125.193

Subnet 3

Subnet 3

11000000.10101001.01111101.11-111111

192.169.125.255


In Table 3-5, you can see the skeleton of the mappings within a 24-bit network address (192.169.125) that is subnetted using a mask of 255.255.255.192. A complete table would have 256 entries and would consume several pages. Rather than subject you to so exhaustive a review of this topic, I've opted for this approach. Each subnet is identified in the left column. The middle column presents the binary string for a specific address within the specified subnet, and the last column gives a decimal translation.

Walking through this table, you can see each subnet's base address. A base address is the starting pointthe first address within a subnet. The base address for the entire block is easy to find, because all 8 bits in the host address octet are set to 0. Each subnet also has a base address and features a host address of 0s. However, subnet host addresses in Table 3-5 contain only 6 bits. Logically, then, each subnet (00, 01, 10, and 11 in this case) begins with the base host address of 000000. Each subnet also ends with the same highest possible address value: 111111. Incrementing beyond this number results in an increment in the subnet field, so that's an unmistakable sign of the boundaries between subnets.

Viewing the subnet and host addresses as a single, concatenated field lets you appreciate the mathematical beginning and ending points of each subnet. The symbols indicate a sequential incrementing of the host field by one binary digit at a time from 000001 through 111110.

Using a 3-Bit Mask

Continuing with our examination of subnetting Class C type networks (24 network address bits), a 3-bit subnetwork mask offers a different combination of subnets and hosts. The mask is 255.255.255.224 because you have the 3 highest-order bits of the last octet to use in defining subnets, so 128 + 64 + 32 = 224. Under strict classical rules, this mask enables the definition of six usable subnets, each with 30 usable host addresses. These are identified in Table 3-6.

Table 3-6. Subnetting with a 3-Bit Mask in a 24-Bit Network
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.000-00000

192.169.125.0

Subnet 1

11000000.10101001.01111101.001-00000

192.169.125.32

Subnet 2

11000000.10101001.01111101.010-00000

192.169.125.64

Subnet 3

11000000.10101001.01111101.011-00000

192.169.125.96

Subnet 4

11000000.10101001.01111101.100-00000

192.169.125.128

Subnet 5

11000000.10101001.01111101.101-00000

192.169.125.160

Subnet 6

11000000.10101001.01111101.110-00000

192.169.125.192

Subnet 7

11000000.10101001.01111101.111-00000

192.169.125.224


As is evident in Table 3-6, the base address in each subnet represents an increase of 32 addresses from the previous subnet's starting point. This is because 32 is the decimal equivalent of the lowest-value bit allocated to the subnet mask. Table 3-7 builds on this example by mapping out the following significant addresses within a subnet:

  • The first, or base, address

  • The first assignable address

  • The last, or broadcast, address

The base address has a host address field of all 0s, the broadcast address has a host address field of all 1s, and the assignable addresses have a host address field populated by a combination of 1s and 0s.

Table 3-7. Mapping Significant Addresses with a 3-Bit Mask
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.000-00000

192.169.125.0

Subnet 0

11000000.10101001.01111101.000-00001

192.169.125.1

Subnet 0

Subnet 0

11000000.10101001.01111101.000-11111

192.169.125.31

Subnet 1

11000000.10101001.01111101.001-00000

192.169.125.32

Subnet 1

11000000.10101001.01111101.001-00001

192.169.125.33

Subnet 1

Subnet 1

11000000.10101001.01111101.001-11111

192.169.125.63

Subnet 2

11000000.10101001.01111101.010-00000

192.169.125.64

Subnet 2

11000000.10101001.01111101.010-00001

192.169.125.65

Subnet 2

Subnet 2

11000000.10101001.01111101.010-11111

192.169.125.95

Subnet 3

11000000.10101001.01111101.011-00000

192.169.125.96

Subnet 3

11000000.10101001.01111101.011-00001

192.169.125.97

Subnet 3

Subnet 3

11000000.10101001.01111101.011-11111

192.169.125.127

Subnet 4

11000000.10101001.01111101.100-00000

192.169.125.128

Subnet 4

11000000.10101001.01111101.100-00001

192.169.125.129

Subnet 4

Subnet 4

11000000.10101001.01111101.100-11111

192.169.125.159

Subnet 5

11000000.10101001.01111101.101-00000

192.169.125.160

Subnet 5

11000000.10101001.01111101.101-00001

192.169.125.161

Subnet 5

Subnet 5

11000000.10101001.01111101.101-11111

192.169.125.191

Subnet 6

11000000.10101001.01111101.110-00000

192.169.125.192

Subnet 6

11000000.10101001.01111101.110-00001

192.169.125.193

Subnet 6

Subnet 6

11000000.10101001.01111101.110-11111

192.169.125.223

Subnet 7

11000000.10101001.01111101.111-00000

192.169.125.224

Subnet 7

11000000.10101001.01111101.111-00001

192.169.125.225

Subnet 7

Subnet 7

11000000.10101001.01111101.111-11111

192.169.125.255


As you saw in Table 3-5, you cross over from one subnet into the other whenever you hit an all-1s address. The next address becomes an all-0s address in the next-higher subnet number. The decimal values offer scant clues to those who are not intimately familiar with subnetting as to the identity of a subnet's base, initial, or broadcast address.

Using a 4-Bit Mask

The next way you can slice up a 24-bit network address is with a 4-bit subnet mask. Subnets of this size are identified with the mask 255.255.255.240 because the first 4 bits of the last octet are used for subnet identification. Mathematically, 240 = 128 + 64 + 32 + 16. Table 3-8 identifies the mathematically possible subnets that such a mask lets you create.

Table 3-8. Subnetting with a 4-Bit Mask in a 24-Bit Network
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.0000-0000

192.169.125.0

Subnet 1

11000000.10101001.01111101.0001-0000

192.169.125.16

Subnet 2

11000000.10101001.01111101.0010-0000

192.169.125.32

Subnet 3

11000000.10101001.01111101.0011-0000

192.169.125.48

Subnet 4

11000000.10101001.01111101.0100-0000

192.169.125.64

Subnet 5

11000000.10101001.01111101.0101-0000

192.169.125.80

Subnet 6

11000000.10101001.01111101.0110-0000

192.169.125.96

Subnet 7

11000000.10101001.01111101.0111-0000

192.169.125.112

Subnet 8

11000000.10101001.01111101.1000-0000

192.169.125.128

Subnet 9

11000000.10101001.01111101.1001-0000

192.169.125.144

Subnet 10

11000000.10101001.01111101.1010-0000

192.169.125.160

Subnet 11

11000000.10101001.01111101.1011-0000

192.169.125.176

Subnet 12

11000000.10101001.01111101.1100-0000

192.169.125.192

Subnet 13

11000000.10101001.01111101.1101-0000

192.169.125.208

Subnet 14

11000000.10101001.01111101.1110-0000

192.169.125.224

Subnet 15

11000000.10101001.01111101.1111-0000

192.169.125.240


This size of mask enables the creation of 14 usable subnets (16 total that are mathematically definable) with 14 usable host addresses each. By now, it should be relatively obvious that you find subnet base addresses by counting in multiples of the decimal value set by the lowest-order bit in the subnet. In a 4-bit subnet, that value is 16. This is reinforced by the Decimal Translation column in Table 3-8.

Given that the tables are getting quite lengthy, and because I'm confident that you get the idea, it isn't necessary to continue mapping the base, initial, and broadcast addresses of each subnet. That would run up the page count, and other topics need to be covered. Therefore, the remainder of this section continues mapping out the base addresses of different-sized subnets in both binary and decimal numbers but foregoes the additional step of mapping out the progression of addresses across the sequential subnets.

Using a 5-Bit Mask

A 5-bit subnet mask is the inverse of a 3-bit mask. In other words, instead of enabling the creation of six subnets of 30 hosts each, a 5-bit mask offers you 30 subnets of six hosts each. The mask for this size of subnet is 255.255.255.248, because 128 + 64 + 32 + 16 + 8 = 248. Table 3-9 gives the breakdown of this mask, as well as the decimal translation of its subnet base addresses.

Table 3-9. Subnetting with a 5-Bit Mask in a 24-Bit Network
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.00000-000

192.169.125.0

Subnet 1

11000000.10101001.01111101.00001-000

192.169.125.8

Subnet 2

11000000.10101001.01111101.00010-000

192.169.125.16

Subnet 3

11000000.10101001.01111101.00011-000

192.169.125.24

Subnet 4

11000000.10101001.01111101.00100-000

192.169.125.32

Subnet 5

11000000.10101001.01111101.00101-000

192.169.125.40

Subnet 6

11000000.10101001.01111101.00110-000

192.169.125.48

Subnet 7

11000000.10101001.01111101.00111-000

192.169.125.56

Subnet 8

11000000.10101001.01111101.01000-000

192.169.125.64

Subnet 9

11000000.10101001.01111101.01001-000

192.169.125.72

Subnet 10

11000000.10101001.01111101.01010-000

192.169.125.80

Subnet 11

11000000.10101001.01111101.01011-000

192.169.125.88

Subnet 12

11000000.10101001.01111101.01100-000

192.169.125.96

Subnet 13

11000000.10101001.01111101.01101-000

192.169.125.104

Subnet 14

11000000.10101001.01111101.01110-000

192.169.125.112

Subnet 15

11000000.10101001.01111101.01111-000

192.169.125.120

Subnet 16

11000000.10101001.01111101.10000-000

192.169.125.128

Subnet 17

11000000.10101001.01111101.10001-000

192.169.125.136

Subnet 18

11000000.10101001.01111101.10010-000

192.169.125.144

Subnet 19

11000000.10101001.01111101.10011-000

192.169.125.152

Subnet 20

11000000.10101001.01111101.10100-000

192.169.125.160

Subnet 21

11000000.10101001.01111101.10101-000

192.169.125.168

Subnet 22

11000000.10101001.01111101.10110-000

192.169.125.176

Subnet 23

11000000.10101001.01111101.10111-000

192.169.125.184

Subnet 24

11000000.10101001.01111101.11000-000

192.169.125.192

Subnet 25

11000000.10101001.01111101.11001-000

192.169.125.200

Subnet 26

11000000.10101001.01111101.11010-000

192.169.125.208

Subnet 27

11000000.10101001.01111101.11011-000

192.169.125.216

Subnet 28

11000000.10101001.01111101.11100-000

192.169.125.224

Subnet 29

11000000.10101001.01111101.11101-000

192.169.125.232

Subnet 30

11000000.10101001.01111101.11110-000

192.169.125.240

Subnet 31

11000000.10101001.01111101.11111-000

192.169.125.249


A mask of 255.255.255.248 in a fixed-length subnet environment is probably pushing the point at which the subdivision of address space is becoming ridiculousat least as far as subnets for end users are concerned. With just six usable host addresses, the value of a subnet becomes questionable for such a purpose. This is particularly true because, in a fixed-length subnetting scheme, the subnet size selected must be able to accommodate the largest of the subnets.

Using a 6-Bit Mask

If a 5-bit mask bordered on ridiculous, surely a 6-bit fixed-length subnet mask is lunacy. Yet FLSM rules let you implement such a scheme. The subnet mask for this scheme in a 24-bit network is 255.255.255.252. Mathematically, you arrive at that value because 128 + 64 + 32 + 16 + 8 + 4 = 252. In binary terms, that mask is 11111111.11111111.11111111.11111100. In other words, not many bits are left for host addresses. In terms of Classical IP rules, with this scheme you could create 62 usable subnets, each bearing just two usable host addresses.

Don't worry. I have no intention of dragging you through a multipage table, exhaustively perusing the incremental binary math of this scheme. Instead, I'll treat you to a very abridged version that still adequately demonstrates the limited utility of such a scheme. Table 3-10 presents the abridged data, again using the 24-bit network address 192.169.125.0.

Table 3-10. Subnetting with a 6-Bit Mask in a 24-Bit Network
 

Binary Network Plus Subnet Address

Decimal Translation

Base

11000000.10101001.01111101.00000000

192.169.125.0

Subnet 0

11000000.10101001.01111101.000000-00

192.169.125.0

Subnet 1

11000000.10101001.01111101.000001-00

192.169.125.4

Subnet 2

11000000.10101001.01111101.000010-00

192.169.125.8

Subnet 3

11000000.10101001.01111101.000011-00

192.169.125.12

Subnet 4

11000000.10101001.01111101. 000100-00

192.169.125.16

Subnet 5

11000000.10101001.01111101. 000101-00

192.169.125.20

Subnet 6

11000000.10101001.01111101. 000110-00

192.169.125.24

Subnet 63

11000000.10101001.01111101.111111-00

192.169.125.252


NOTE

The /30 subnet might seem thoroughly useless, given how few addresses it contains. However, it might well be one of the most commonly used subnet masks. It is ideally suited for addressing serial links (such as T1 or other point-to-point leased lines) on routers. However, the usefulness of so small a subnet is apparent only when you escape the confines of fixed-length subnetting schemes.


Notice in Table 3-10 how the base address of each subnet increments by four addresses. As demonstrated earlier in this chapter, half of these mathematically possible addresses remain reserved under the classical rules of subnetting. The four addresses are 00, 01, 10, and 11. The 00 and 11 addresses function as the subnet identity and the broadcast address for each subnet, respectively. Thus, a 6-bit mask gives you a plethora of subnets, but with a dearth of host addresses per subnet.

Hopefully, this review of the trade-offs between subnet and host addresses has helped reinforce the basic concepts of subnetting and has given you a lot to think about. I wouldn't want to leave you with the misimpression that only Class C (24-bit) networks can be subnetted. Far from it! Any network address can be subnetted. To help demonstrate this, the next section briefly examines how subnetting works with a Class B network (16-bit) address.

Subnetting a 16-Bit Network Address

A Class B network contains many more addresses than a Class C. As explained in Chapter 2, the Class B network uses 16 bits of its address space for host identification and the other 16 bits for network identification. That means that each Class B network can uniquely identify 65,536 possible endpointsquite a difference from the paltry 256 available within a Class C network!

Having so much address space to work with means that there is a lot more flexibility in forming subnets. However, the limitations of FLSM still apply. The limitations of FLSM were noticeable in a Class C network, and those same limits are experienced on an even greater scale within a Class B network.

Apart from the sheer size difference, subnetting a Class B network with FLSM is much like subnetting a Class C. Some of the basic rules that must be adhered to are that a minimum of 2 bits and a maximum of 14 may be used to identify subnets. These 16 bits are the third and fourth octets in the 32-bit IPv4 address. Implicit in these statements is that a subnet address can spill over between two of an IP address's octets. That can make things a bit trickier to understand, but it's an extension rather than a violation of the subnetting rules you are now so familiar with. These bits also must start out with the highest-order bits of the host address in order to ensure that the extended network prefix remains a contiguous block of bits. Table 3-11 presents the basic set of trade-offs between the number of subnets and the number of hosts per subnet that you can create with FLSM.

Table 3-11. Subnetting a 16-Bit Network

Number of Bits in the Network Prefix

Subnet Mask

Number of Usable Subnet Addresses

Number of Usable Hosts Per Subnet

2

255.255.192.0

2

16,382

3

255.255.224.0

6

8,190

4

255.255.240.0

14

4,094

5

255.255.248.0

30

2,046

6

255.255.252.0

62

1,022

7

255.255.254.0

126

510

8

255.255.255.0

254

254

9

255.255.255.128

510

126

10

255.255.255.192

1,022

62

11

255.255.255.224

2,046

30

12

255.255.255.240

4,094

14

13

255.255.255.248

8,190

6

14

255.255.255.252

16,382

2


The sheer size of a Class B network should be evident from Table 3-11. A mask of 255.255.192.0, for example, yields only two usable subnets, but each one offers 16,382 host addresses.

Using 172.16.0.0 as our sample network block, let's look at how it can be subnetted and which addresses become base subnet addresses. Without subjecting you to the tedium of an exhaustive exploration of how a 16-bit network can be subnetted, refer to Table 3-12 for a much-condensed version. This table simply demonstrates the key difference between subnetting a Class B versus a Class C. That difference lies in how the address bits are allocated between network, subnet, and host fields. I have again followed the convention of using bold for the network address, bold italic for the subnet address, and plain text for the host address to help you more readily appreciate the boundaries between these fields.

Table 3-12. Subnetting a 16-Bit Network

Subnet Mask

Binary Equivalent

255.255.192.0

11111111.11111111.11-000000.00000000

255.255.224.0

11111111.11111111.111-00000.00000000

255.255.240.0

11111111.11111111.1111-0000.00000000

255.255.248.0

11111111.11111111.11111-000.00000000

255.255.252.0

11111111.11111111.111111-00.00000000

255.255.254.0

11111111.11111111.1111111-0.00000000

255.255.255.0

11111111.11111111.11111111.00000000

255.255.255.128

11111111.11111111.11111111.1-0000000

255.255.255.192

11111111.11111111.11111111.11-000000

255.255.255.224

11111111.11111111.11111111.111-00000

255.255.255.240

11111111.11111111.11111111.1111-0000

255.255.255.248

11111111.11111111.11111111.11111-000

255.255.255.252

11111111.11111111.11111111.111111-00


Table 3-12 has some interesting points to consider. First, did you notice that there is a mask of 255.255.255.128? That mask would be illegal in a Class C network, because it would use only 1 bit for subnet identification. However, in the much larger Class B space, that mask indicates that 9 bits (not 1) are being used for subnet identification. Thus, you should recognize that the legality of any particular mask depends directly on the size of the network address block.

Other interesting subnets are last five, which range from 255.255.255.192 through 255.255.255.252. These masks are identical to those found in Class C networks. However, recognize that in this particular case, each one has 8 additional bits being used for subnet identification. Thus, the number of hosts per subnet may remain equal to the Class C counterpart, but the number of possible subnets that can be created is much greater. Again, the size of the network address block being subnetted has tremendous implications for the subnetting scheme. Context is everything.

At this point in the chapter, you are probably very tired of staring at tables of numbers. Fear not: You're almost through with this topic. The remainder of this chapter points out some of the specific weaknesses and inefficiencies inherent in FLSM. These are the reasons why FLSM has become functionally obsolete. You can still use it, but as you will learn in the next chapter, you have better options.




IP Addressing Fundamentals
IP Addressing Fundamentals
ISBN: 1587050676
EAN: 2147483647
Year: 2002
Pages: 118
Authors: Mark Sportack

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