The optional network you will create in this network will be a DMZ, which is short for the DeMilitarized Zone. The idea of a DMZ is to have a network where some traffic is allowed to pass and some traffic is not. Typically, traffic in the DMZ is allowed to pass to and from the internet but not to other internal networks. Traffic is allowed to pass from internal networks to the DMZ. Thus, the flow of traffic looks like this:

Internet <<>> DMZ << Internal networks

Unsafe internet traffic, for example, is allowed to enter a web server in the DMZ. LAN traffic is allowed to enter the DMZ as well, for example, if someone on the LAN wants to access the web server as well. However, the key lies in the fact that no DMZ traffic is allowed to access the internal networks.

To configure a DMZ, you will need at least one spare interface, and you will have to have added it using the procedure outlined in the Identifying and assigning interfaces recipe. We will assume that you have added at least one such interface (named OPT1).