Standards and Tools for Computer Network Diagrams?

_kabel asks: "Recently I've been asked to draw a map of my company's computer network. Gathering the data to do this isn't the problem though - it's drawing it all. This is no easy task, as it contains hundreds of hosts in many locations around the world, linked together with quite a variety of methods (modem, ISDN, ADSL, VPN over Internet and more.) So I have two questions: (1) Are there any standards on computer network diagrams (i.e. the symbols used for the various devices and connections) and (2) are there any tools already available to assist in this?" I know Visio has the capabilities and the symbols to do this, but what about the free-software diagramming tools?