Cisco CCNA

Using SubnetZero to Get Around the Rules

Keeping in mind the caveats listed in the preceding section, you can configure Cisco routers to use the bottom subnet so that you gain one more subnet out of your subnetting efforts. To enable the use of the bottom subnet, use the ip subnet-zero global command:

Router#conf t

Router(config)#ip subnet-zero

If you forget to configure this, the router will "complain" when it comes time to assign an address to an interface. The following is an attempt to configure an interface with an address from a bottom subnet on a router without the ip subnet-zero command (notice the output Bad mask):

Router(config)#int s0

Router(config-if)#ip address 192.168.1.2 255.255.255.224 Bad mask /27 for address 192.168.1.2

Because the broadcast address for the top subnet is the same as the broadcast address to the entire major net, deploying the top subnet with such classful routing protocols as RIP and IGRP is not recommended. This is not a problem for classless routing protocols, such as OSPF and EIGRP.

A Word on Semantics

For the remainder of this book, the term network defines a general service of TCP/IP communication, as in the "corporate network" or "enterprise network." This is also known as an organization's intranet and is usually built of campus networks and wide-area networks. The term major net refers to a specific IP address space that follows classful addressing, and subnet refers to an address space that is extracted from the major net with the subnetting procedure covered earlier in "Subnetting a Classful Address Space."