Concept Guide
Next Header (8 bits)
The Next Header eld identies the next header’s type. If an Extension header is used, this eld contains the type of Extension header (as
shown in the following table). If the next header is a transmission control protocol (TCP) or user datagram protocol (UDP) header, the
value in this eld is the same as for IPv4. The Extension header is located between the IP header and the TCP or UDP header.
The following lists the Next Header eld values.
Value Description
0 Hop-by-Hop option header
4 IPv4
6 TCP
8 Exterior Gateway Protocol (EGP)
41 IPv6
43 Routing header
44 Fragmentation header
50 Encrypted Security
51 Authentication header
59 No Next Header
60 Destinations option header
NOTE: This table is not a comprehensive list of Next Header eld values. For a complete and current listing, refer to the Internet
Assigned Numbers Authority (IANA) web page at .
Hop Limit (8 bits)
The Hop Limit eld shows the number of hops remaining for packet processing. In IPv4, this is known as the Time to Live (TTL) eld and
uses seconds rather than hops.
Each time the packet moves through a forwarding router, this eld decrements by 1. If a router receives a packet with a Hop Limit of 1, it
decrements it to 0 (zero). The router discards the packet and sends an ICMPv6 message back to the sending router indicating that the
Hop Limit was exceeded in transit.
IPv6 Routing
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