Network Organization

Properly organizing your network addressing process grows more difficult as the size of your network grows. Setting up network addressing for a Class C network with fewer than 254 devices is simple. Setting up addressing for a large, worldwide company with a Class A network and many different users can be extremely complex. If your company has fewer than 254 hosts (meaning any device that requires an IP address, including computers, printers, routers, switches, and other devices) and all your workgroups can share information, a single Class C network will be sufficient.

Subnetting

Within Class A and B networks, there can be separate networks called subnets. Subnets are considered part of the host portion of an address for network class definitions. For example, in the 128. Class B network, you can have one computer with an address of 128.10.10.10 and another with an address of 128.10.200.20; these computers are on the same network (128.10.), but they have different subnets (128.10.10. and 128.10.200.). Because of this, communication between the two computers requires either a router or a switch. Subnets can be helpful for separating workgroups within your company.

Often subnets can be used to separate workgroups that have no real need to interact with or to shield from other groups information passing among members of a specific workgroup. For example, if your company is large enough to have its own HR department and payroll section, you could put those departments' hosts on their own subnet and use your router configuration to limit the hosts that can connect to this subnet. This configuration prevents networked workers who are not members of the designated departments from being able to view some of the confidential information the HR and payroll personnel work with.

Subnet use also enables your network to grow beyond 254 hosts and share IP addresses. With proper routing configuration, users might not even know they are on a different subnet from their co-workers. Another common use for subnetting is with networks that cover a wide geographic area. It is not practical for a company with offices in Chicago and London to have both offices on the same subnet, so using a separate subnet for each office is the best solution.

Subnet Masks

Subnet masks are used by TCP/IP to show which part of an IP address is the network portion and which part is the host. Subnet masks are usually referred to as netmasks. For a pure Class A network, the netmask would be 255.0.0.0; for a Class B network, the netmask would be 255.255.0.0; and for a Class C network, the netmask would be 255.255.255.0. Netmasks can also be used to deviate from the standard classes.

By using customized netmasks, you can subnet your network to fit your needs. For example, your network has a single Class C address. You have a need to subnet your network. Although this isn't possible with a normal Class C subnet mask, you can change the mask to break your network into subnets. By changing the last octet to a number greater than zero, you can break the network into as many subnets as you need.

For more information on how to create customized subnet masks, see Day 6, "The Art of Subnet Masking," in Sams Teach Yourself TCP/IP Network Administration in 21 Days. This chapter goes into great detail on how to create custom netmasks and explains how to create an addressing cheat sheet for hosts on each subnet. You can also browse to the Linux Network Administrator's Guide and read about how to create subnets at http://www.tldp.org/LDP/nag2/index.html.

Broadcast, Unicast, and Multicast Addressing

Information can get to systems through three types of addresses: unicast, multicast, and broadcast. Each type of address is used according to the purpose of the information being sent, as explained here:

• Unicast Sends information to one specific host. Unicast addresses are used for telnet, FTP, SSH, or any other information that needs to be shared in a one-to-one exchange of information. Although it is possible that any host on the subnet/network can see the information being passed, only one host is the intended recipient and will take action on the information being received.

• Multicasting Broadcasts information to groups of computers sharing an application, such as a video conferencing client or online gaming application. All the machines participating in the conference or game require the same information at precisely the same time to be effective.

• Broadcasting Transmits information to all the hosts on a network or subnet. Dynamic Host Configuration Protocol (DHCP) uses broadcast messages when the DHCP client looks for a DHCP server to get its network settings, and Reverse Address Resolution Protocol (RARP) uses broadcast messages for hardware address to IP address resolution. Broadcast messages use .255 in all the host octets of the network IP address. (10.2.255.255 will broadcast to every host in your Class B network.)