Both for learning about networks and in my work as a system and network admin, I found Wireshark (and Ethereal before that) one of the most useful tools around.

Both for learning about networks and in my work as a system and network admin, I found Wireshark (and Ethereal before that) one of the most useful tools around. I once diagnosed a networking problem a friend had by getting him to install it on his laptop and record a packet dump of his traffic, then send it to me via email.