Technical Library

Why would I need multiple Networks?
How do I add Multiple Networks?
Is there anything else that I should know?

Why would I need multiple networks?

There are many reasons for needing more than one network number.

The most common reason for having multiple networks is because you have more nodes than available IP addresses.

For example, if you have 300 hosts on your LAN and you want to connect all of them, you would not have enough address space with only one class-C network. Since a class-C network consists of only 254 host addresses, you would need at least 2 class-C network numbers. (See note (a) below.)

Another common reason for setting up multiple networks is to separate local traffic from incoming traffic.

If you have a dual Ethernet router, you can set up two separate LANs routed over the Internet where the traffic goes either to a "public" side or a "private" side. The "private" network normally would contain your local workstations that are used to browse the web, ftp to remote sites, read mail, etc. The public side would house your Web server, FTP server, Gopher server, SMTP server, etc. By setting up packet filters in the router, inbound IP traffic can be confined to your public network. Remember that you must allow traffic to return to the private network. (See note (b) below.)

Having multiple sites with one Internet connection is another common reason for having multiple networks routed.

For example, if you have sites in LA, NYC, and Albany and you want to connect all three sites to the Internet through the Internet connection at your Albany site, you would need each of the sites to have its own unique routed network number (and proper routing set up internally).

How do I add multiple networks?

To add multiple networks you need to do two things.

First, separate the networks physically. Each network must be on its own segment and numbered properly.

Secondly, the proper routing must be set up. You must add static routes for each of the additional networks that are being routed, or you can set up dynamic routing if your topology is too unwieldy for a static routing table. For specific router syntax and setup examples refer to your router documentation.

Is there anything else that I should know?

(a) You should not put multiple networks on one segment. If you only have one network interface on your router, you can set up an internal router with multiple network cards. You would have to route the additional network(s) to the interface of the internal router that is on the same segment as the Internet router.

(b) If you are going to set up a public and a private LAN, you will have to set up IP packet filters on the serial interface as well as both network interfaces. Also, keep in mind that you should allow connections from the "private" side to the "public" side, but you should not allow any connections to be established from outside the local network.