CCnnnCCa2CCCCCCCCa*

Another way to look at this is to realize that whitespace is ignored in a pack/unpack template and that template specifiers can be quantified. The original unpack template might have been written as
    'C C n n n C C a2 C4 C4 a*'
to slightly clarify just what is going where and that in, e.g.,
    ..., @ip_src[0..3], ... = unpack( ... C4 ... );
the array slice receives four unsigned bytes.

Hey gmargo, that's exactly what I was looking for. Is there a site you got that from? I'd also like to be able to figure out how to decode other protocols as well.

I typed it in from the front inside cover of my Stevens book (TCP/IP Illustrated Volume 1).

Wikipedia has it too: http://en.wikipedia.org/wiki/IPv4, but the real gospel is RFC791: http://www.ietf.org/rfc/rfc791.txt.

Update: Other protocol standards you might like:
RFC 768 User Datagram Protocol
RFC 792 Internet Control Message Protocol
RFC 793 Transmission Control Protocol

I have that same book and just see the IP header format. How do you know what letters to use in the (un)pack template just by looking at the header format?