the server "listens" on one/many network protocols/ports, so clearly your client-side must pick [at least one of] those [via network/aliasses part of sql config tool]
some SQL installs (especially SQLEXPRESS) deliberately default to not listening on network ports
- ie just listen for local memory-based (ie lightweight procedure call)
so if your SSMS client-side is using NP and IP but server-side is only using local, you will fail
note that if you open up IP or other protocols you will need to cycle the SQLservice to take effect
- it does tell you that on the dialog box but you'd be suirprised how many people don't read it !
HTH
Dick