There is a graph APP called OmniGrafle for macs, and like in visio , you need to look for stencils. I started using this app resently so I am not fully familiar with its full capability in comparison with visio but it can do network diagrams . It is said is capable of importing visio .vsd in its newer versions.
Although this thread is a bit old, this is still a problem for Mac users as there's no version of Visio for it yet. I gave up on trying to run Windows and mac because there are now some very good alternatives to Visio for Mac. Almost all of them now import and export Visio files although not all import stencils but that's just a matter of time. Some of them are cloud based but with Office 365 and some of Visio now cloud based it doesn't really make much difference IMHO.