IP protocol number

IP protocol number

The numeric identification of the upper layer protocol that an IP packet should be sent to. The number is stored in the header that is prefixed to an IP packet. Note that the IP protocol number is not the same as the port number (see TCP/IP port), which refers to a higher level, such as the application layer. Following are some of the common IP protocol numbers.
IPProtocol   #   PurposeTCP        6   Transport type
  UDP       17   Transport type

  ICMP       1   Control message
  IGMP       2   Multicast

  EGP        8   Exterior gateway
  IGP        9   Interior gateway
  RSVP      46   Reserve bandwidth

  IP         4   Encapsulation (IP in IP)
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It sees only an IP protocol number and must be either allowed or blocked, without knowing any more details regarding the content of the packet.