11-7
IP Routing—Configuring Static Routes
Overview
When you use prefix lengths in this way, the bit length becomes, in a sense,
part of the address. 172.16.0.0 /20 is a different network than 172.16.0.0 /16.
The second is the network address for the entire class B network, while the
first is a network that includes only hosts from 172.16.0.1 to 172.16.15.254.
Therefore, when you define routes to variable-length subnets, you must
always be careful to specify the correct bit length. If a router thinks that it
knows a route to network 172.16.0.0 /16 when the route should actually be to
172.16.0.0 /20, it may misroute traffic to the other fifteen 20-bit networks in
the 172.16.0.0/16 range.
Routing Table
A routing table stores the following information for each network that the
router knows how to reach:
■ destination network address
■ subnet mask
■ next-hop address
■ forwarding interface
■ metric
■ administrative distance
Destination Network Address and Subnet Mask
The destination network address and subnet mask identify the route. When a
router receives a packet, it matches the packet’s destination IP address to a
network address in the routing table. The subnet mask defines how many bits
the router examines when matching the two addresses. For example, a routing
table entry for 172.16.0.0 with a subnet mask 255.255.0.0 refers to all packets
destined to IP addresses of which the first 16 bits are 172.16.
If a packet matches more than one entry, the router uses the more-specific
route (the route with a longer subnet mask), which it assumes is more accurate
for that packet.
The subnet mask condenses the routing table: an individual router’s table need
not include a separate entry for each host or subnet in the 172.16.0.0/16
network when the next hop to all these destinations is the same. Routers
nearer a particular destination may include more specific entries that allow
them to forward traffic to individual networks that have been subdivided from
a larger network.
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