IPv6 Header Structure Format and Fields Explained (2023)

This tutorial explains the structure, format, and fields of the IPv6 header. Learn what extension headers are in IPv6 and how they are used.

The IPv6 header is a streamlined version of the IPv4 header. It provides new features while retaining the core concepts that made IPv4 popular.It eliminates the fields that are rarely used, improves the remaining fields, and adds an additional field to provide better support for real-time traffic.In this tutorial, we will discuss these changes in detail along with the structure and format of the IPv6 header.

To learn about the IPv4 header and its fields in detail, you can check the following tutorial.

IPv4 Header Structure and Fields Explained

The IPv6 header structure

The following image shows the basic structure of the IPv6 header.

IPv6 Header Structure Format and Fields Explained (1)


This field defines the version of the packet. In IPv6, the value of this field is set to 6. The size of this field is 4 bits.

Traffic class

This field defines how the packet should be handled through the intermediate devices.This field replaces the IPv4 type of service or differentiated services field. The size of this field is 8 bits.Same as the IPv4, the first 6 bits are defined as the DSCP field and the last 2 bits are defined as ECN.DSCP and ECN are respectively documented in RFC 2474 and RFC 3168.

The DSCP field indicates the type or priority of the packet that the router should follow when making therouting or forwarding decision. If a router fails to forward a packet, it uses the ECN field of the packet tosend an Explicit Congestion Notification back to the original sender.

Flow label

A flow is a sequence of packets that are exchanged between a source and destination in a single session.A source can exchange data in multiple sessions simultaneous. To identify the sequence and session of the packet,the flow label field is used. In addition to identifying the packet's sequence and session, this field is also usedto specify how the packet should be handled by intermediate routers.

The size of this field is 20 bits. For default router handling, the value of this field is set to 0.Intermediate routers use the packet's source address, a destination address, and flow label to distinguish between different flows.

Payload length

This field specifies the total length of the payload in bytes. This length includes the length of extension headers.

The size of this field is 16 bits. With 16 bits, this field can indicate a payload of up to 65535 bytes in length.A payload greater than 65535 bytes is known as a jumbo payload. IPv6 supports the jumbo payload.For the jumbo payload, the value of this field is set to 0 and the jumbo payload option is used in the Hop-by-Hop options extension header.

Since the base IPv6 header is fixed in length, a router can easily calculate the total length of thepacket by adding the payload length and the base header length.

Next header

This field indicates the type of the first extension header. If an extension header is not present,it specifies the protocol in the upper-layer PDU. Protocols are identified with standard 8 bits values defined by IANA.If both an extension header and the upper-layer protocol are not available,the value 59 is used in this field. The size of this field is 8 bits.

The following lists the common next header values and their meanings.

Value Protocol header, or option
0 Hop-by-Hop Options header
17 UDP
41 Encapsulated IPv6 header
43 Routing header
44 Fragment header
50 Encapsulating Security Payload header
51 Authentication header
58 ICMPv6
59 No next header
60 Destination Options header

Hop limit

This field sets the lifetime of the packet. Every time a router forwards a packet, it decrements the value of this field by 1.If the value reaches zero, the packet is discarded. The size of this field is 8 bits.

Source IP Address

This field specifies the IPv6 address of the sender of the packet.

Destination IP Address

This field indicates the IPv6 address of the intended recipient(s) of this packet.

Extension headers

This is a new concept introduced in IPv6. In this concept, rarely used IP options are made optional and a newfield called extension header is defined to add optional headers. The extension header field not only optimizes therarely used options of IPv4 but also allows developers to define new options without modifying the structure of the IPv6 header.Any new option can easily be added to the extension header field.

Instead of adding all options, the sender node can select the required options and add them to the extension header field.Intermediate routers also do not need to process all options in the extension header. They can skip the options that are not intended for them.Options in this field are referred to as extension headers. Extension headers are added between the base header and the transport-layer header in a packet.

Let's discuss the most common options or extension headers.

Hop-by-Hop options header

If used, this header always appears on the first position in the field. This header contains two options thatevery intermediate router must examine and process. These options are the Jumbo payload option and the Router alert option.

The jumbo payload option is used to send a packet that contains a payload longer than 65535 bytes.If this option is enabled, it indicates that the packet contains the jumbo payload. Not all routers support the jumbo payload.If an intermediate router cannot forward a packet that contains the jumbo payload,it discards the packet and sends an ICMPv6 error message to the sender.

The router alert option is used to instruct intermediate routers that information inside thepacket is intended to be viewed and processed by them even though the packet is addressed to some other node.

Routing header

The routing header is used to send the packet to the destination through a specific route.This option allows the sender device to specify multiple destination addresses in a list.The initial destination of the packet is not the same as the final destination of the packet,but rather the first address in the list contained in the Routing header.When the destination node which address is set in the first place of the list receives the packet,it resends the packet to the next node which address is set in the second place of the list.This process continues until the packet reaches its final destination.

Fragment header

Fragmentation allows a sender device to send a large packet in multiple smaller packets.In IPv4, fragment-related options are part of the main header. In IPv6, these options are moved to the fragment header.These options are a fragment offset, more fragments flag, and identification for the fragment.

Destination Options Header

This header contains options and information for the destination node. Currently, only padding options are specified for this header.Padding options are used to fill out the header on a 64 bits boundary.

Authentication Header

This header is used to calculate a cryptographic checksum on some parts of the IPv6 base header, extension headers, and payload.

Encapsulating Security Payload Header

This header indicates that the payload is encrypted. If the payload is encrypted it also includesenough information for the authorized destination node to decrypt it. If used, this header will always be the last in the extension header.

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By ComputerNetworkingNotes Updated on 2022-04-09 11:12:06 IST

ComputerNetworkingNotes Networking TutorialsIPv6 Header Structure Format and Fields Explained


What is IPv6 explain with header format? ›

IPv6 header format comprises Version, traffic class, flow label, Payload Length, next header, hop limit, Source address, and destination address. IPv6 fixed header is 40 bytes in length. IPv6 extension header stores the rarely used or unnecessary information and the necessary information is stored in the fixed header.

What are the fields in the IPv6 header? ›

A Next Header field in the IPv6 header indicates the next extension header. Within each extension header is a Next Header field that indicates the next extension header. The last extension header indicates the upper layer protocol (such as TCP, UDP, or ICMPv6) contained within the upper layer protocol data unit.

Which is the correct format for an IPv6 header? ›

IPV6 header format is of 40 bytes in length, contains information essential to routing and delivery, consist of 8 fields, Version, Traffic Class, Flow Label, Payload length, next header, HOP limit, Source address and destination address, where each has its own features and provides essential data required to transmit ...

What is IPv6 Please explain the structure of IPv6 packet header? ›

An IPv6 packet is the smallest message entity exchanged using Internet Protocol version 6 (IPv6). Packets consist of control information for addressing and routing and a payload of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers.

What are the format of headers? ›

The general HTTP header format contains colon-separated name - value pairs in the header field. Each of the name-value pair end with a carriage return (CR) and a line feed (LF) character sequence. Empty fields at the end of each header indicate the end of the header.

What are the features and format of IPv6? ›

Changes from IPv4 to IPv6 are described in the following categories: Expanded routing and addressing capabilities – IPv6 increases the IP address size from 32 bits to 128 bits to support more levels of addressing hierarchy. In addition, IPv6 provides a greater number of addressable nodes.

How many basic header field does IPv6 contain? ›

IPv6 Basic Header. An IPv6 basic header is fixed as 40 bytes long and has eight fields. Each IPv6 packet must have an IPv6 basic header that provides basic packet forwarding information, and which all devices parse on the forwarding path.

What is the first field in an IPv6 header? ›

The first field of an Internet Control Message Protocol (ICMP) packet specifies the message type.

How many bits are in IPv6 field? ›

IPv6 uses 128-bit (2128) addresses, allowing 3.4 x 1038 unique IP addresses. This is equal to 340 trillion trillion trillion IP addresses. IPv6 is written in hexadecimal notation, separated into 8 groups of 16 bits by the colons, thus (8 x 16 = 128) bits in total.

Can you explain IPv6 header? ›

IPv6 headers have one Fixed Header and zero or more Optional (Extension) Headers. All the necessary information that is essential for a router is kept in the Fixed Header. The Extension Header contains optional information that helps routers to understand how to handle a packet/flow.

What is an example of an IPv6 address format? ›

An IPv6 address is represented as eight groups of four hexadecimal digits, each group representing 16 bits The groups are separated by colons (:). An example of an IPv6 address is: 2001:0db8:85a3:0000:0000:8a2e:0370:7334.

What is the size and format of IPv6 address? ›

The IPv6 address size is 128 bits. The preferred IPv6 address representation is: x:x:x:x:x:x:x:x , where each x is the hexadecimal values of the eight 16-bit pieces of the address. IPv6 addresses range from 0000:0000:0000:0000:0000:0000:0000:0000 to ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff .

What are the 4 types of formatting? ›

To help understand Microsoft Word formatting, let's look at the four types of formatting:
  • Character or Font Formatting.
  • Paragraph Formatting.
  • Document or Page Formatting.
  • Section Formatting.
11 Apr 2022

What are the formatting formats? ›

Formatting refers to the appearance or presentation of your essay. Another word for formatting is layout. Most essays contain at least four different kinds of text: headings, ordinary paragraphs, quotations and bibliographic references. You may also include footnotes and endnotes.

What is a header field? ›

Header fields are colon-separated key-value pairs in clear-text string format, terminated by a carriage return (CR) and line feed (LF) character sequence. The end of the header section is indicated by an empty field line, resulting in the transmission of two consecutive CR-LF pairs.

What are the two main formats in which IPv6 address is represented? ›

Representation of IPv6 Addresses. IPv6 addresses are 128 bits in length and written as a string of hexadecimal digits. Every 4 bits can be represented by a single hexadecimal digit, for a total of 32 hexadecimal values (016 [00002] through f16 [11112]).

Which header field name and position changed in IPv6? ›

Table 4-2. IPv4 Header Fields and Corresponding IPv6 Equivalents
IPv4 Header FieldIPv6 Header Field
ProtocolReplaced by the IPv6 Next Header field.
Header ChecksumRemoved in IPv6. The link layer has a checksum that performs bit-level error detection for the entire IPv6 packet.
9 more rows
15 Jun 2012

Why was the IPv6 address format created? ›

IPv6 Address is the new generation IP address that is mainly developed to overcome IPv4 exhaust and its limitations. As you know, IPv4 Addresses were limited and exhaused shortly. For the new technologies, more IP addresess needed and for this need a new IP version has developed.

What is the size of an IPv6 main header? ›

The header structure of an IPv6 packet is specified in RFC 2460. The header has a fixed length of 40 bytes. The two fields for source and destination addresses each use 16 bytes (128 bits), so there are only 8 bytes for general header information.

How many fields are in a complete format of an IPv6 address? ›

An IPv6 address is 128 bits in length and consists of eight, 16-bit fields, with each field bounded by a colon. Each field must contain a hexadecimal number, in contrast to the dotted-decimal notation of IPv4 addresses. In the next figure, the x's represent hexadecimal numbers.

What are the 3 main types of IPv6 addressing? ›

The three types of IPv6 addresses are: unicast, anycast, and multicast. Unicast addresses identify a single interface.

What are the parts of IPv6? ›

An IPv6 address is split into two parts: a network and a node component. The network component is the first 64 bits of the address and is used for routing. The node component is the later 64 bits and is used to identify the address of the interface.

What is IPv6 range? ›

IPv6 addresses range from 0000:0000:0000:0000:0000:0000:0000:0000 to ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff . In addition to this preferred format, IPv6 addresses might be specified in two other shortened formats: Omit leading zeros. Specify IPv6 addresses by omitting leading zeros.

What is IPv6 used for? ›

The primary function of IPv6 is to allow for more unique TCP/IP address identifiers to be created, now that we've run out of the 4.3 billion created with IPv4. This is one of the main reasons why IPv6 is such an important innovation for the Internet of Things (IoT).

How many bytes is IPv6? ›

The IPv6 address is 128 bits (16 bytes).

What is the format of an IP address IPv4 and IPv6? ›

The IPv4 is a 32-bit address, whereas IPv6 is a 128-bit hexadecimal address. IPv6 provides a large address space, and it contains a simple header as compared to IPv4. IPv6 increases the IP address from 32 bits to 128 bits to support larger address demands.

How do I write an IPv6 address? ›

An IPv6 address is 128 bits in length and is written as eight groups of four hexadecimal digits. Each group is separated from the others by colons (:) as shown in figure 1. Hexadecimal characters are not case sensitive, therefore an address can be written either in uppercase or lowercase, both are equivalent.

What is 0.0 0.0 in IPv6? ›

0.0 with netmask 0.0. 0.0 represents the default route. The same concept is also applied to IPv6, address 0:0:0:0:0:0:0:0 with netmask all 0s represents the default route.

What is IPv6 notation? ›

IPv6 is the latest version of IP that is suitable for large-scale IP networks. IPv6 supports a 128-bit address to allow 2128, or approximately 3.4×1038 addresses while IPv4 supports only 232 addresses.

What are three important features of IPv6? ›

It offers the following features:
  • Larger Address Space. In contrast to IPv4, IPv6 uses 4 times more bits to address a device on the Internet. ...
  • Simplified Header. ...
  • End-to-end Connectivity. ...
  • Auto-configuration. ...
  • Faster Forwarding/Routing. ...
  • IPSec. ...
  • No Broadcast. ...
  • Anycast Support.

What is IPv6 simple definition? ›

IPv6 (Internet Protocol version 6) is a set of specifications from the Internet Engineering Task Force (IETF) that is essentially an upgrade of IP version 4 (IPv4), a category of IP addresses in IPv4-based routing.

What is the header format of IPv4? ›

An Internet Protocol version 4 packet header (IPv4 packet header) contains application information, including usage and source/destination addresses. IPv4 packet headers contain 20 bytes of data and are normally 32 bits long. A packet is a network communication data unit containing fixed or variable lengths.

What are the 4 types of IPv6 addresses? ›

IPv6 unicast address assignment consists of the following forms: Aggregate global unicast address. Neutral-interconnect unicast address. NSAP address.

Why is IPv6 used? ›

The IPv6 protocol can handle packets more efficiently, improve performance and increase security. It enables internet service providers to reduce the size of their routing tables by making them more hierarchical.

Where is IPv6 used? ›

IPv6 (Internet Protocol version 6) is the sixth revision to the Internet Protocol and the successor to IPv4. It functions similarly to IPv4 in that it provides the unique IP addresses necessary for Internet-enabled devices to communicate.

What is the format of IPv4 header and describe the significance of each field? ›

IPV4 header format is of 20 to 60 bytes in length, contains information essential to routing and delivery, consist of 13 fields, VER, HLEN, service type, total length, identification, flags, fragmentation offset, time to live, protocol, header checksum, source IP address, Destination IP address and option + padding, ...

What is IPv4 header and IPv6 header? ›

A packet with additional information which transmits from source to destination that uses Internet Protocol version 4 is called IPv4 header. A packet with additional information which transmits from source to destination that uses Internet Protocol version 6 is called IPv6 header.

What is the options field in IPv4 header? ›

The IPv4 header is variable in size due to the optional 14th field (options). The IHL field contains the size of the IPv4 header; it has 4 bits that specify the number of 32-bit words in the header. The minimum value for this field is 5, which indicates a length of 5 × 32 bits = 160 bits = 20 bytes.

What is the format of IP address answer? ›

The IP address is a 32-bit number that uniquely identifies a network interface on a machine. An IP address is typically written in decimal digits, formatted as four 8-bit fields separated by periods. Each 8-bit field represents a byte of the IP address.

What is the format of an IP address and give an example? ›

An IP address is a string of numbers separated by periods. IP addresses are expressed as a set of four numbers — an example address might be Each number in the set can range from 0 to 255. So, the full IP addressing range goes from to

How many IPv6 addresses are there? ›

IPv6 uses 128-bit (2128) addresses, allowing 3.4 x 1038 unique IP addresses. This is equal to 340 trillion trillion trillion IP addresses.

What is the length of an IPv6 header? ›

IPv6 does not include a Header Length field because the IPv6 header is always a fixed length of 40 bytes.


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