Understanding the IPv6 Address and IPv6 Packet Headers

This is (Sixth Article) in continuation of our last one article, a new series of networking articles. In this segment, we are going to discuss the idea of the IPv6 addresses and how the IPv6 packets are developed.


Let us begin with a simple question: Why go for IP version 6 (IPv6), when IPv4 is already standardized and is widely used across the globe? IPv4 has a 32-bit address scheme. Due to the exponential growth of Internet users, IPv4 is running out of address space.

In IP version 6 (IPv6), 128-bit address space (in place of 32 bits of IPv4) and minimum 40 byte IP header (in place of a minimum of 20 bytes of IPv4) has been proposed. IPv6 is for coping with increasing demands of Internet access. IP addressing for IPv6 is 128 bits or 16 bytes.

With IPv4, quality voice, video, and multimedia services are not being serviced. On the other hand, growth in voice IP traffic is increasingly huge. IPv6 has been proposed to address the stated limitations of IPv4. IPv6 does so with added features, as given below.

1. 128-bit address size ensures address space of about 2^128 compared to theoretical maximum of only 2^32 in IPv4. IPv6 is believed to support trillions of networks.

2. Provision for Quality Of Service (QoS) support to real-time services like voice and video and support to mobility.

3. Flow Label and Priority in the header of IPv6 facilitates support of real-time data.

The header of IPv6 is different from that of IPv4 in many aspects, as follows:

(a) IPv6 has a fixed header size (320 bits), not at all like IPv4. Options and padding are variable fields in IPv4. These have been eliminated in IPv6. This makes IPv6 act in the Asynchronous Transfer Mode (ATM) cell in contrast with IPv4.

(b) There is no header checksum in IPv6. This change is made dependent on the experience that there is no requirement for checksum at each node.

(c) There is no Hop-By-Hop segmentation method in IPv6. Thus, there is no fragment offset, flags and identification fields in IPv6 and also there is no requirement for a header length field.

(d) Type of Service (TOS) field of IPv4 is also eliminated in IPv6. This elimination is based on understanding, as the TOS field is not really used in IPv4.

IPv6 defines Six Extension Headers: Hop-By-Hop Options, Routing, Fragment, Encrypted Security Payload, Authentication, and Destination Options.

In fact, IPv6 can be compared with IPv4 on the basis of two significant parameters, in particular, address space and IP header simplification. With IPV6, exponential increment in Internet clients after some time with the arrangement of in abundance address space of 2^128. With a simplified header, IPv6 is inclined to give interactive media (voice, video, and information) services. It is accordingly the future substitution of IPv4.

Features of IPv6

1. IPv6 has a fixed header which is of fixed 53 bytes. What is the upside of having a fixed size? Let us think about the postal system. If envelopes are of variable sizes and loads, handling gets delayed, as the procedure incorporates estimating weight, confirming postal stamps required by weight, etc. If there were fixed-sized and weighted envelopes. Postal handling would take considerably less time and exertion. Correspondingly, with a fixed header, node handling delay is diminished. This makes IPv6 increasingly reasonable for continuous services like voice, video and interactive media.

2. A 128-bit address space in IPv6, rather than a 32-bit address space in IPv4, is to guarantee that there will be no exhaustion of Internet address space.

3. Other than Unicast and Multicast, IPv6 has the arrangement for anycast tending to that permits a packet routed to an anycast address to be conveyed to any of the gatherings of hosts.

4. Arrangement of Extension Header in IPv6 meets the prerequisites of such things as a checksum, security options and so on.

5. Flow-level and Priority headers are used to help real-time services. By assigning higher priority to real-time packets, the need for time affectability is reestablished.

6. IPv6 gives security support that could inevitably be viewed as its greatest bit of leeway.

IPv6 Headers (Source: Practical Guide to Advanced Networking)

Functions of IPv6 headers

1. Version Field (4 bits) contains the version number of Internet Protocol. Starting now, there are just two versions: 4 and 6. For version 6, this field is six, which makes four bits of this field as 0110.

2. Priority (4 bits) Field is used to show the priority of the packet. Four bits in the field characterize 2 (=16) priority levels. The initial eight priority levels (from 0000 to 0111) are allotted for the services requiring congestion control. If that blockage happens, traffic is backed off.

Different priority levels under the initial eight levels are utilized as 0 for no need, 1 for background traffic, 2 for unattended exchange like emails or messages, 3 stays reserved, 4 for mass exchange like File Transfer Protocol (FTP), 5 stays reserved, 6 for Telnet, X-windows, etc, and 7 for control traffic, for example, Simple Network Management Protocol (SNMP) and routing protocols.

Higher priority levels (from eight to fifteen) are used for services that don’t back off. Priority levels eight and fifteen are, individually, for traffic that is generally ready to be discarded of on clog and for traffic that is least ready to be discarded of on blockage.

3. Flow level (24 bits) is used to identify different data flow characteristics, which may be assigned by the source and can be used to label packets. Packet labels may be required to provide special handling of packets by IPv6 routers.

4. Payload length (16 bits) field indicates the total size of the payload (which tells routers about the amount of information a particular packet) in bytes of the IP packet  that excludes header fields.

5. Next header (8 bits) field indicates the header that follows the IP header. The next header can either be one of the optional extension headers used by IP or the header for an upper-layer protocol such as UDP or TCP.

6. Hop Limit (8 bits) field is used to limit the maximum number of hops that a packet is permitted to move in the system before it arrives at its destination. If after moving the maximum number of hops, as allotted by this field, the packet doesn’t arrive at the destination, it is disposed of from the system. Every router reduces the hop limit by one before releasing the packet to the system. When the hop limit arrives at zero, it is deleted by the detecting router. Hop limit point of IPv6 is actually time-to-live (TTL) in IPv4.

7. Source and Destination Address : The IP address of the original source of the packet is called source address. IP address of the destination of the packet is called destination address. Each is 128 bits. Three primary groups of IP addresses in IPv6 are unicast. multicast and anycast.

The unicast address is for a particular host. A unicast packet is identified by its unique single address for a single interface network interface card (NIC) and is transmitted point-to-point. A multicast address is for all hosts of a particular group to receive the datagram. The anycast address is to address a number of interfaces on a single multicast address.

The notion for writing IPv6 address is that each double-byte of the 16-byte field is separated by a colon. Bytes are written in hexadecimal symbols. Thus, one example of the address could be 1234:ABCD:56EF:00 and 79:0000:0000:9876:456C.

This is enough for this section, we have talked about the idea of the IPv6 addresses and how the IPv6 packet is developed. In the next article, we will take a look at IPv6 Extension Headers and ICMP.

If you have any suggestions or thoughts, just comment down below.

Related Concepts:

1. Understanding the Communication and Internet Beginning

2. Understanding the idea of Internet, OSI Model and TCP Model

3. Understanding the TCP Layer and TCP Packet Header

4. Understanding the IP address and IPv4 Packet Header

5. Understanding the UDP Headers, ARP and RARP Protocol

Biplab Das

Biplab Das

My name is Biplab Das. I’m a writer, Blogger, Programmer, ICT tutor and a part-time website developer whose childhood obsession with science fiction never quite faded. A quarter-century later, the technology that I coveted as a kid is woven into the fabric of everyday life. I’ve spent the past years to learn these technologies, i recently published a book on computer science fundamentals. People say smartphones are boring these days, but I think everyone is beginning to take this wonderful technology marvel for granted.
Keep Digging Technology.

Sign up for our Newsletter

Sign Up for our Newsletter for daily Updates !