IP Addressing Guidelines

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IP Addressing Guidelines

As discussed earlier, a network administrator can choose to use any IP address he or she likes for an internal TCP/IP network (intranet). However, the following information should be kept in mind as these are notable exceptions:

The network ID of 127 is reserved as the loopback address. It is also used in diagnostics.
A network ID of all 1s or all 0s is never assigned to an individual network.
A host ID of all 1s or all 0s is never assigned to an individual host.
The value 255.255.255.255 represents the broadcast address.

The IP address 127.b.c.d, with b,c and d each being any number between 0 and 255, represents a software loopback address. Any packet sent to this address will be returned to the application without transmission to the network. That is, the information is returned to host from which it originates, without being sent out to the network. The packet is being copied from the transmit to receive buffer on the same host. Hence the name “loopback address.” This address can be used as a check to see that TCP/IP software has been installed correctly.

For example, executing a ping 127.0.0.1 command on a Windows NT server will request a packet to be sent to itself. A return of this packet will imply a successful installation of TCP/IP. However, the return of this packet will not necessarily imply a successful configuration of TCP/IP. See Chapter 16, “Troubleshooting Microsoft TCP/IP,” for more information.

Host ID values of all 0s are not assigned to individual hosts, because these addresses represent the network itself. For example, the IP address of 207.21.32.0 represents the Class C network 207.21.32. Similarly, the IP address of 10.0.0.0 represents the class A network 10. The IP address containing all 1s in the Host ID segment of the address, the address is known as a directed broadcast. For example, the IP address of 207.21.32.255 would be the address a packet is sent to if it is to be received by all hosts on the Class C network 207.21.32. Similarly, a packet sent to an address of 10.255.255.255 would be received by all hosts in the Class A network 10.

A network ID of all 0s is not defined. As seen in Table 3.3, the valid range of Class A networks is 1 to 126, and not 0 to 126. Similarly, a network ID containing all 1s is not defined.

The address 255.255.255.255 is referred to as the local broadcast. This type of broadcast address can be used in a local area network, or intranet, where a broadcast will never cross a router boundary.

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