There isn't a standard for network diagrams like there is with UML, however there are common icons for common networking devices (routers, switches, computers, databases, firewalls, users, etc).
Cisco has a set of icons it publishes free for use (I'm sure there's a license agreement) and they make the claim that their icons are:
globally recognized and generally accepted as standard for network
icon topologies.
Most diagrams include text describing what the icon is, so as long as you use something that looks like the device (no comment about Cisco's router icon) and has an accurate description, you should be OK.