An IP protocol number is an 8-bit value in an IP header that identifies the next protocol or header carried after it. For example, 6 identifies TCP and 17 identifies UDP. It is not an IP address, an IP version, or a TCP/UDP port.
What an IP protocol number means
In IPv4, the selector is in the header’s Protocol field. It tells the receiving system what kind of next-level protocol follows the IP header, so the packet can be handed to the appropriate handler. The field is 8 bits wide, allowing 256 possible bit patterns (28); that count is calculated from the field width, not quoted as a separate IANA statistic.
IANA describes the field as identifying “the next level protocol” and notes that IPv6 calls its corresponding field “Next Header.” The [IANA Protocol Numbers registry] lists assigned values and references. Its page was last updated March 9, 2026; consult it for current assignments.
Common IP protocol numbers
| Number | Assignment |
|---|---|
| 1 | ICMP |
| 6 | TCP |
| 17 | UDP |
These values are assignments in IANA’s [Protocol Numbers registry]. The number identifies a protocol in the IP header; it does not identify an application or connection by itself.
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Is an IP protocol number the same as a port?
No. The protocol number is in the IP header and identifies the next-level protocol, such as TCP or UDP. A port number is used within TCP or UDP to distinguish application endpoints. For instance, an IP packet’s protocol number can identify TCP; the TCP header then contains its own source and destination port fields. They belong to different headers and answer different questions.
How IPv6 uses the protocol number
IPv6 names its corresponding 8-bit selector the Next Header field. As RFC 8200 puts it, it “Identifies the type of header immediately following the IPv6 header.” The selector uses the same IANA number space as IPv4’s Protocol field.
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IPv6 can also place extension headers between its base header and an upper-layer header. In that case, the base header’s Next Header value identifies the first extension header; that extension header has its own Next Header value identifying what follows. The chain is read one header at a time until the next protocol or header is reached. See [RFC 8200, section 3] for the standard definition.
| IP version | Selector field | Width | What it identifies |
|---|---|---|---|
| IPv4 | Protocol | 8 bits | The next-level protocol after the IPv4 header |
| IPv6 | Next Header | 8 bits | The immediately following header; extension headers can form a chain |
Protocol number versus IP version number
These numbers describe different things. The IP version field is 4 bits wide: its value indicates whether the packet uses IPv4 (4) or IPv6 (6). The protocol number or Next Header value instead identifies what follows the relevant IP header. IANA’s [IP Version Numbers registry] covers version-field assignments.
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- IP version: Which IP format is being used, such as IPv4 or IPv6.
- Protocol number / Next Header: Which protocol or header comes next within that IP packet.
Where to check a value
Use the [IANA Protocol Numbers registry] as the authoritative reference for assignments and related standards. Registry assignments can change, so verify a value there when accuracy matters rather than relying solely on a short list of common examples.
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