This fast-paced video session from JavaZone functions as an introduction to networking fundamentals for programmers:

This session will start with the underlying understanding of networking at a low level. At this level we will discuss, IP, MAC, ARP, DNS and DHCP. As we walk up an abstraction level, we discuss sockets, NAT, gateways and firewalls along with the use of TCP and UDP. Spending some time at this layer can make network developers more productive, as we look at tools which help us answer the question, "who owns this port?", "where is this packet going?" and "What is my latency and why?"

Networks for Programmers from JavaZone on Vimeo.