Cisco® CCNP Route Exam Notes : Ipv4 And Ipv6 Fragmentation

1. Network Principles

1.2 IPv4 and IPv6 fragmentation

IP Fragmentation: is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size. The fragments are reassembled by the receiving host.

IPv4 Fragmentation: An IP v4 header format is given below:

IP v4 header format

The following list describes the function of each header field.

  • Version: This Field defines the version of IP. It is Static 4 bit value.
  • Header Length: This Field defines the length of the datagram . It is 4 bit value.
  • Type of Service: It is 8 bit value. It is used tell the network how to treat the IP packet. These bits are generally used to indicate the Quality of Service (QoS) for the IP Packet.
  • Packet Length: 16 bit value indicating the size of the IP Packet in terms of bytes. This gives a maximum packet size of 65536 bytes.
  • Identification: 16 bit field used for reassembling the packet at the destination.
  • Flags: It is 3 bits value. It indicates if the IP packet can be further fragmented or not and if the packet is the last fragment or not of a larger transfer.
  • Fragment offset: 13 bit value used in the reassembly process at the destination.
  • Time to Live: 8 bit value telling the network how long an IP packet can exist in a network before it is destroyed.
  • Protocol: 8 bit value used to indicate the type of protocol being used (TCP, UDP etc.).
  • Header checksum: It is 16 bit value. It is used to indicate errors in the header only. Every node in the network has to check and re-insert a new checksum as the header changes at every node.
  • Source address: 32 bit value representing the IP address of the sender of the IP packet.
  • Destination address: 32 bit value representing the IP address of the packets final destination.
  • Options: Options are not required for every datagram. They are used for network testing and debugging.
  • Padding: Variable size bit field. These bits are used to ensure a 32 bit boundary for the header is achieved.

IPv6 Fragmentation: IPv6 packet is 320 bits or 40 octets long. It will have basic packet header, and optional extension header. The next header field within an extension header points to the next header in the chain. The following figure shows the fields that appear in the IPv6 header and the order in which the fields appear.

Note that there is no fragment information in Ipv6 header. In order to send a packet larger than the PMTU, an IPv6 node may fragment a packet at the source and have it reassembled at the destination.


Note that in IPv6, only hosts can fragment whereas in IPv4, both hosts and routers can fragment.

IPv6 Fragmentation has always been discouraged as the reassembly is computationally expensive and inefficient. Additionally, there are some security concerns with fragmentation.

Ipv6 Header Format

The following list describes the function of each header field.

  • Version - 4-bit version number of Internet Protocol = 6.
  • Traffic class - 8-bit traffic class field.
  • Flow label - 20-bit field. Flow label is a new field in the IPv6 header. A 6-to-4 tunnel works similarly to a manual tunnel, except that the tunnel is set up automatically. 6-to-4 tunnels use IPv6 addresses that concatenate 2002::/16 with the 32-bit IPv4 address of the edge router, creating a 48-bit prefix.
  • Payload length - 16-bit unsigned integer, which is the rest of the packet that follows the IPv6 header, in octets.
  • Next header - 8-bit selector. Identifies the type of header that immediately follows the IPv6 header. Uses the same values as the IPv4 protocol field.
  • Hop limit - 8-bit unsigned integer. Decremented by one by each node that forwards the packet. The packet is discarded if the hop limit is decremented to zero.
  • Source address - 128 bits. The address of the initial sender of the packet.
  • Destination address - 128 bits. The address of the intended recipient of the packet. The intended recipient is not necessarily the recipient if an optional routing header is present.

The extension header may include the following:

  • Hop-by-Hop options
  • Destination options
  • Routing (specifies intermediate routers that the route must include forcing an administratively defined path)
  • Fragment (Used to divide packets that are too large for the maximum unit (MTU) )
  • Authentication and Encapsulating Security Payload (ESP)

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