Can anyone tell me what I need to do to the firewall in order to make a shared printer available to other machines on my network?

I have the printer set up and have tested it with the firewall switched off, but as soon as I restart the firewall, the printer is inaccessible.

I thought that all traffic on the internal network was allowed, but it seems that I need to create an explicit rule in order to get the shared printer working. Unfortunately, I haven't been able to figure out how to do that yet.