Sample project ideas:

Write a NAT in Java

The following project is based on the C program which performs Net Address Translation (NAT).

You can get NAT as part of the "IP Filter" package, which includes accounting and a packet-filtering firewall. According to the web page, it "has been tested and run on <list of irrelevant other OSes> Linux 2.0.31-35." Go to http://www.cyber.com.au/cyber/product/ipfilter/ and click on "IP Filter technical information."