What is an IP Protocol Number?

Forever a question that is asked when configuring TCP/IP security of some sort.

In the Internet Protocol version 4 (IPv4) there is a field, called "Protocol", to identify the next level protocol.  This is an 8 bit field.

Firewalls can look at this field and use it in a packet filter.
 

Below you can click to see a capture of a ping packet (ICMP) clearly showing that it is Protocol Number 1 (0x01)

Just like Port Numbers, Protocol Numbers are assigned.  Here is a table with some common Protocol Numbers you may need to know.

 If you wish to see the whole list then visit - Assigned Internet Protocol Numbers at http://www.iana.org/assignments/protocol-numbers or have a look at the inline frame below...

I hope this all helps?

Matt Rowston

KB Keywords:  TCP/IP, IP, Protocol Number, Firewall, packet filter.