System.Net.Sockets ? not in add reference list ?

hi everyone

I want to add a reference to System.Net.Sockets in visual studio 2003. Its not in the add reference list.

How can I add it when its not in the list? or where can I find it?

/tortillasuses System.Net.Sockets (c#)
imports System.Net.Socketd (vb.net)