Unable to connect without specifying port number

Question

I have a strange situation on the client side.  While connecting to sqlserver remotely, and when the instance is set up on a non-default port (not 1433), I am unable to establish a connection unless I specify the port number.  This might seem normal
at first, but when my colleagues establish a remote connection (I am in an office) they do not need to specify a port number.  I've seen other questions posted on the forum related to the port number, but the difference is that our DB configuration works
for everyone except myself, thus it seems like a client side issue.
For example, "dbserver\instancename" works for everyone else, but fails for me unless I specify "dbserver\instancename,PortNum".  This is true through SQL management studio, through code, or through anything.
So my question is, what should I look for on my machine as the culprit?  At first glance, there is not much different between my machine and the others.  We're all on Win 7, we use active directory, and we all seem to belong to the same groups
(give or take).  My machine has much more installed in the way of dev tools (visual studio 2013, all of the dotnet frameworks, etc).  The DB is SQL 2005.
I have no access to the DB server, nor will the DBAs help me troubleshoot--they claim it's a client side problem.  So I am hoping I can isolate the problem on my end.  Thanks in advance.

Answer 1

>Do you have any firewall configured on your machine? Here's how the the initial handshake happens
- when you try to connect to SQL Server named instance without specifying a port number, SQL send the information about port number back on the UDP port 1434 and then the connection succeeds. If your UDP port 1434 on the client is closed, the client connection
wouldn't know which port to connect to and will fail. 
I've disabled all antivirus software and stopped the windows firewall service.  No change.