Use Of Subnet Zero; Classless Routing - Cisco Catalyst 3750 Software Configuration Manual

Configuring IP Addressing

Use of Subnet Zero

Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise
if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as
255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.
You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Command
Step 1
configure terminal
Step 2
ip subnet-zero
Step 3
end
Step 4
show running-config
Step 5
copy running-config startup-config
Use the no ip subnet-zero global configuration command to restore the default and disable the use of
subnet zero.

Classless Routing

By default, classless routing behavior is enabled on the switch when it is configured to route. With
classless routing, if a router receives packets for a subnet of a network with no default route, the router
forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C
address spaces used to simulate a single, larger address space and is designed to relieve the pressure on
the rapidly depleting Class B address space.
In Figure
discarding the packet, the router forwards it to the best supernet route. If you disable classless routing
and a router receives packets destined for a subnet of a network with no network default route, the router
discards the packet.
Figure 28-2 IP Classless Routing
128.20.1.0
Catalyst 3750 Metro Switch Software Configuration Guide
28-6
Purpose
Enter global configuration mode.
Enable the use of subnet zero for interface addresses and routing updates.
Return to privileged EXEC mode.
Verify your entry.
(Optional) Save your entry in the configuration file.
28-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
128.0.0.0/8
128.20.4.1
128.20.0.0 IP classless
128.20.2.0
128.20.4.1
128.20.3.0
Host
Chapter 28 Configuring IP Unicast Routing
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