Ask HN: Projects/resources to learn more about networking?

I'm about 3/4 done with a networking course I'm currently taking at my local college for the summer, which covers most of the material you need to know for the CCNA. I'm really enjoying the course and have learned a lot of stuff about the OSI model, IP addressing, static and dynamic routing, WAN links, VLANs, spanning tree, all kinds of stuff. Today I just finished another module where I learned about IP addressing services like DHCP and NAT.

I would like to use my programming experience to create a project that uses some of the knowledge I've learned so far. I was thinking about creating my own software router (emulator or something, not sure if that's too ambitious of a goal though) or maybe a simple VPN, or something interesting that I can have fun developing.

Does anyone know of any resources to point me in this direction?