Is an IP protocol number the same as a port number?

No. The protocol number is an 8-bit field in the IP header that names the next-layer protocol, such as 6 for TCP or 17 for UDP. Port numbers belong to TCP and UDP themselves and identify the application endpoint.

Where does the protocol number live in IPv6?

IPv6 has no Protocol field. The same values appear in the Next Header field of the IPv6 base header (and of each extension header), forming a chain that ends at the upper-layer protocol.

What protocol numbers do IPsec tunnels use?

IPsec uses protocol 50 for the Encapsulating Security Payload (ESP) and protocol 51 for the Authentication Header (AH). Firewalls must permit these numbers directly, not just UDP/TCP ports, unless NAT-Traversal wraps ESP in UDP 4500.

Why is OSPF assigned its own protocol number?

OSPF runs directly over IP as protocol 89 rather than over TCP or UDP, so it carries no port. Routers identify and filter OSPF traffic by this protocol number.

What is protocol number 41 used for?

Protocol 41 is IPv6 encapsulation — it carries an IPv6 packet inside an IPv4 packet, the basis of 6in4 tunnels used to reach IPv6 over an IPv4-only path.

IP Protocol Numbers — Gera Tools

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Every IP packet carries an 8-bit value that names the protocol immediately above IP: 6 for TCP, 17 for UDP, 1 for ICMP, and dozens more. In IPv4 this is the Protocol field; in IPv6 it is the Next Header value. This reference lets you search the IANA registry by number or by name so you can read packet captures, write firewall rules and understand tunnelling.

How it works

The field is a single byte (0–255), so it identifies the next-layer protocol but not an application — that distinction belongs to TCP/UDP ports. In IPv6 the same numbering threads through the Next Header chain: each extension header points to the next, and the final value names the upper-layer payload (for example 6 for TCP or 58 for ICMPv6).

Search by the exact number or by a keyword. Routing protocols like OSPF (89) and EIGRP (88), tunnelling protocols like GRE (47) and IPv6-in-IPv4 (41), and the IPsec pair ESP (50) / AH (51) all appear here because they sit directly on IP with no port of their own.

Tips and examples

Common values worth memorising:

1   ICMP        17  UDP        47  GRE        51  AH
2   IGMP        41  IPv6 (6in4) 50 ESP        58  ICMPv6
6   TCP         89  OSPF       132 SCTP       112 VRRP

When a firewall blocks an IPsec tunnel even though UDP 500 (IKE) is open, the usual culprit is a missing rule for protocol 50 (ESP). Likewise, a GRE tunnel needs protocol 47 permitted end-to-end — opening TCP/UDP ports does nothing for it.