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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Addressing Concepts and Issues

IP Address Size, Address Space and "Dotted Decimal" Notation 1 2 3 IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)

Implications of Including the Network ID in IP Addresses

The fact that the network identifier is contained in the IP address is what partially facilitates the routing of IP datagrams when the address is known. Routers look at the network portion of the IP address to determine first of all if the destination IP address is on the same network as the host IP address. Then routing decisions are made based on information the routers keep about where various networks are located. Again, this is conceptually similar to how the area code is used by the equivalent of “routers” in the phone network to switch telephone calls. The host portion of the address is used by devices on the local portion of the network.

Since the IP address can be split into network ID and host ID components, it is also possible to use either one or the other by itself, depending on context. These addresses are assigned special meanings. For example, if the network ID is used with all ones as the host ID, this indicates a broadcast to the entire network. Similarly, if the host ID is used by itself with all zeroes for the network ID, this implies an IP address sent to the host of that ID on “the local network”, whatever that might be.

It is the inclusion of the network identifier in the IP address of each host on the network that causes the IP addresses to be network-specific. If you move a device from one network to a different one the network ID must change to that of the new network. Therefore, the IP address must change as well. This is an unfortunate drawback that shows up most commonly when dealing with mobile devices.

IP Address Size, Address Space and "Dotted Decimal" Notation 1 2 3 IP Addressing Categories (Classful, Subnetted and Classless) and IP Address Adjuncts (Subnet Mask and Default Gateway)