IP Protocol Number List: nmap-protocols

This file maps the one-byte IP protocol number in the IP header into the corresponding protocol name. Example 14.7 is a typical excerpt.

Example 14.7. Excerpt from nmap-protocols
hopopt           0     HOPOPT      # IPv6 Hop-by-Hop Option
icmp             1     ICMP        # Internet Control Message
igmp             2     IGMP        # Internet Group Management
ggp              3     GGP         # Gateway-to-Gateway
ip               4     IP          # IP in IP (encapsulation)
st               5     ST          # Stream
tcp              6     TCP         # Transmission Control
cbt              7     CBT         # CBT
egp              8     EGP         # Exterior Gateway Protocol
[ ... ]
chaos           16     CHAOS       # Chaos
udp             17     UDP         # User Datagram

The first two fields are the protocol name or abbreviation and the number in decimal format. Nmap doesn't care about anything after the protocol number. It is used for IP protocol scanning, as described at the section called “IP Protocol Scan (-sO)”. Fewer than 140 protocols are defined and users almost never modify this file. The raw data is made available by the IANA at http://www.iana.org/assignments/protocol-numbers.