It would appear by your situation that you have to use NAT. You have more than one machine in the network, and are only receiving one DHCP-assigned address... that spells NAT lol.
Also, I see and support ocicat's defense of wireshark. While it may be a security issue itself, it's also a wonderful learning tool with which to familiarize yourself with L2 networking.
As a last posit- If you are going to use OpenBSD for this project (and I highly suggest you do), you may be interesting in
pfw . This is not to discourage you from the CLI management of pf- but rather to show you that it can be done very well from a web interface.