IPv6 has a field, the Next Header field, that specifies the type of header expected after the main IPv6 header. Although it is similar to the IPv4 Protocol field, there is an additional option.
The initial values for various protocols are specified in RFC 1700, Assigned Numbers, and were later replaced by an online database maintained by the Internet Assigned Numbers Authority (IANA) at www.iana.org/assignments/protocol-numbers/ protocol-numbers.xml. The same values used in the IPv4 Protocol field are used in the IPv6 Next Header field, along with some additional values for IPv6. Some of the most common values for both protocols are 6 for TCP and 17 for UDP.
In a situation when there is only the main IPv6 header and no extension headers, the Next Header field specifies the protocol carried in the data portion of the IPv6 packet. This is similar to the Protocol field in the IPv4 header.
Significant IPv6 Next Header Field Values
| Next Header Field Value | Description | |
| Decimal | Hexadecimal | |
| 0 | 0 | Hop-by-hop options extension header for IPv6 |
| 1 | 1 | Internet Control Message Protocol version 4 (ICMPv4) |
| 2 | 2 | Internet Group Management Protocol version 4 (IGMPv4) |
| 4 | 4 | IPv4 encapsulation |
| 5 | 5 | Internet Stream Protocol (ST) |
| 6 | 6 | Transmission Control Protocol (TCP) |
| 8 | 8 | Exterior Gateway Protocol (EGP) |
| 17 | 11 | User Datagram Protocol (UDP) |
| 41 | 29 | IPv6 encapsulation |
| 43 | 2B | Routing extension header for IPv6 |
| 44 | 2C | Fragment header for IPv6 |
| 46 | 2E | Resource Reservation Protocol (RSVP) |
| 47 | 2F | Generic Routing Encapsulation (GRE) |
| 50 | 32 | Encapsulating Security Payload (ESP) |
| 51 | 33 | Authentication Header (AH) |
| 58 | 3A | Internet Control Message Protocol version 6 (ICMPv6) |
| 59 | 3B | No Next Header for IPv6 |
| 60 | 3C | Destinations options extension header for IPv6 |
| 88 | 58 | Enhanced Interior Gateway Routing Protocol (EIGRP) |
| 89 | 59 | Open Shortest Path First (OSPF) |
Above shows three examples of the IPv6 Next Header field in decimal, indicating the information following the main IPv6 header. Usually, the information after the main IPv6 header is the data or payload, such as a TCP segment. But as the last example above illustrates, the Next Header field can also point to an extension header.
About the Author
Rick Graziani has been an instructor of computer networking and computer science courses at Cabrillo College in Aptos, California since 1994. Rick also teaches networking courses in the Computer Engineering department at the University of California, Santa Cruz and is on the Curriculum Engineering team for Cisco Networking Academy.
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