some IP protocol codes (from Sonicwall Elearning)

Protocol Code                     Protocol Description
1                                          ICMP - internet Control Message Protocol
2                                          IGMP - internet group management protocol
4                                          IP within IP
6                                          TCP - transmission control protocol
17                                         UDP - user datagram protocol
41                                         IPv6
47                                         GRE - generic router ecapsulation (used by pptp)
50                                         IPSEC (ESP - ecapsulating security payload)
51                                          IPSEC (AH - authentication header)