IPv6 Header Field Calculator

About this interactive

The IPv6 main header is always the same size, and most of it is the two 128-bit addresses. The rest holds six fields: Version, Traffic Class (DSCP bits for priority and ECN bits for congestion, as in IPv4's second byte), Flow Label (ties the packets of one flow together), Payload Length, Next Header (names what follows, usually TCP or UDP) and Hop Limit. Two of them trip people up. Payload Length counts only what comes after the main header, unlike IPv4's Total Length, so a packet's full size is its payload length plus the main header's size. Hop Limit does the job of IPv4's Time to Live: each router lowers it by one, and a router that brings it to zero drops the packet. IPv6 has no header checksum and routers do not fragment, which is why its longer header is quicker to process. Seven of the sixteen questions are drawn each run. Any you miss shows the answer and how to work it out.

What you'll learn

Work with the IPv6 main header's fields: its fixed 40-byte size, Payload Length, Hop Limit, Flow Label, Traffic Class and Version
Read a captured header's values the way Wireshark shows them, and compare the header's size with IPv4's

Aligned to

Cisco CCNA

1.9 Describe IPv6 address types

CompTIA Network+

3.4 Given a scenario, implement IPv4 and IPv6 network services

Key terms

IPv6 Header: A fixed-length 40-byte header used in IPv6 packets, containing fields such as version, traffic class, flow label, payload length, next header, hop limit, and source and destination addresses.
Hop Limit: A field in the IPv6 header that starts at a value set by the sender and is decremented by one at each router; when the value reaches zero the packet is discarded and an ICMPv6 error is sent to the source. The hop limit serves the same loop-prevention purpose as the TTL field in IPv4.
Flow Label: A 20-bit field in the IPv6 header used to identify and group related packets that belong to the same traffic flow.
Traffic Class: An 8-bit IPv6 header field used for quality of service and congestion notification, equivalent to the DSCP and ECN fields in IPv4.
Next Header: An IPv6 header field that identifies the type of header immediately following the IPv6 header, typically indicating the Layer 4 protocol such as TCP or UDP.
Maximum Transmission Unit: The largest size of a packet or frame that can be transmitted over a network link without fragmentation, typically 1500 bytes for Ethernet. When packets exceed the MTU, they must be fragmented at Layer 3 or the sender must reduce the packet size via Path MTU Discovery.

Topics

Ipv6 Ipv6 Header Hop Limit Payload Length Flow Label Interactive Calculation

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