Question.

I have three computers on a network. They are part of a windows workgroup.
When I have them plugged into a switch and then the switch into the router, they can communicate fine.
When I plug each of them into the router (eth ports on a bridged network) they can't communicate with each other.

My guess is that I have to set up dns entries for the workgroup names. Is this correct? How do I do so?

Thanks in advance.

Well, bridged interfaces are operating the same as switch from the client point of view.
Make sure that all of them are bridged.

There is no doubt all interfaces are bridged. The problem is when they try to communicate with each other, it isn't by IP address, it is by workgroup name, and that's not resolving.

Since teh communication is by workgroup and not IP, do I need the DNS entries for the workgroup names?

What you are trying to do is NETBIOS over TCP which most definitely uses IP. In the absence of a WINS server, NETBIOS works by devices broadcasting for and electing a Browse Master, this is a device running some version of NETBIOS, usually a Microsoft operating system or Samba on Linux. There is a hierarchy which determines which device becomes the Browse Master, NT Servers are at the top of the hierarchy and Client PCs are usually at the bottom. As the election of the Browse Master is accomplished by broadcasting, this means that it will only work on Bridged or Switched networks, the broadcast packets are not passed though a routed interface. The elected Browse Master is effectively a mini-WINS server running in background servicing the local network, but unable to work in cooperation with WINS servers over a wider routed network. Each time a new device is added to the network this can potentially cause a Browse Master re-election which can be observed as a short burst of intensive broadcast activity. Devices running an operating system version that is high up in the election hierarchy can potentially upset the apple cart if rebooted regularly. In Microsoft operating systems there is a Registry setting that allows you to disable the ability for any given device from becoming a Browse Master, I believe there is also an equivalent setting in Samba. Networks with a number of server level operating systems would do well to disable Browse Master capability on most systems to avoid unnecessary re-elections.

NETBIOS over TCP should work fine on a bridged network without needing to add any entries to DNS. You really only need DNS entries where you want to connect to network shares across a routed network rather than using the host's raw IP address. Note, you can't browse for a network share across routed networks without using WINS servers, but you can still manually enter the host/share name with either the raw IP address or the NETBIOS name if entered into a local DNS.

You say your MT interfaces are bridged, but the symptoms you describe would indicate they actually aren't.

For the MT interfaces to be truly bridged you must create a bridge, add each Ethernet port to the bridge and add the IP address to the bridge, not the Ethernet port.

I didn't realise I had to add the IP address to the bridge. In my case, I have 6 devices that need to plug into 4 ports. How do I enter an IP address in a bridge port when the port plugs into a switch to connect three machines to the network? Can I enter three IP addresses?