IP Addressing
F-4
893-743-A
Addresses and Routing
Addresses make it possible to route and deliver data packets. For example,
when a host sends an Internet data packet to a destination host on the same
network, the packet goes directly to the destination host. If the destination host
and sending host are on different networks, the packet goes to a router.
To determine whether the destination host is on the same network, the sending
host compares the network portions of the destination address and its own
address. If these network numbers are the same, the destination host is on the
same network. If the network numbers are different, the destination host is on
another network, and the data packet must go to a router.
A router has two or more network interfaces onto different networks. The
router’s primary function is to direct packets between these networks,
delivering them to their final destination or to another router. (A router-to-
router transmission is called a hop.)
To begin the routing process, the router examines the network number of the
destination address. Using this number as a key, the router locates applicable
routing information in its routing table. The router uses this routing information
to send the packet to its final destination or to an intermediate destination.
Subnetting
Subnetting is a scheme for imposing a simple hierarchy on hosts on a single
physical network. The usual practice is to use the first few bits in the host
portion of the network addresses for a subnet field. For example, Figure F-4
shows a Class B address with five bits of the host portion used as the subnet
field. The official description of subnetting is RFC 950, Internet Standard
Subnetting Procedure.
Figure F-4. Subnet field in a Class B address
As with the host portion of an address, do not use all zeros or all ones in the
subnet field.
1
14
11
Host
0
1
Network
5
Subnet
1
