Answer to Question #317927 in Computer Networks for Aman

Question

Q2.1: Mention and explain each field of IPv6 header in detail, by taking suitable examples. (5 marks)

Q2.2: Find the first address, last address and the number of addresses of a network whose one

of the IP addresses is 50.28.22.75 / 28. (5 marks)

Answer 1

Q2.1

Version

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 the routing or forwarding decision. If a router fails to forward a packet, it uses the ECN field of the packet to send 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 used to 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 the packet 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.

Q2.2: 1) The number of addresses in the largest sub block ,which requires 120 addresses, is not power of 2, we allocate 128 addresses .The subnet mask for this subnet can be found as 32 -log₂128 =25. The first address in this block is 150.25.0.0/25 ; the last address is 150.25.0.127/25.

2)The number of addresses in the second largest sub block , which requires 60 addresses , is not a power of 2 either, we allocate 64 addresses .The subnet mask for this subnet can be found as $32 - log_{2}64 = 26$ .The first address in this block is 150.25.0.128/26;The last address is 150.25.0.191/26

3) The number of addresses in the smallest sub block ,which requires 10 addresses, is not a power of 2 either we allocate 16 addresses. The subnet mask for this subnet can be found as $32-log_{2}16 = 28$ . The first address in this block is 150.25.0.192/28; The last address is 150.25.0.207/28