Question

Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has the following forwarding table: \begin{tabular}{cc} \hline Prefix Match & Interface \\ \hline 00 & 0 \\ 010 & 1 \\ 011 & 2 \\ 10 & 2 \\ 11 & 3 \\ \hline \end{tabular} For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range.

Short answer

Interface 0: 0-63 (64 addresses); Interface 1: 64-95 (32 addresses); Interface 2: 96-127, 128-191 (32+64 addresses); Interface 3: 192-255 (64 addresses).

Step-by-step solution

Understanding Prefix Matching

In a datagram network, prefix matching is used to determine the next hop for a packet based on the destination address. The router uses the longest prefix matching, which means it matches the longest sequence of bits in the address to decide the forwarding interface.

Determine Address Range for Prefix '00'

The prefix '00' indicates that the first two bits of the host address should be '00'. The remaining 6 bits can vary, meaning the range is from `00000000` to `00111111`. This covers host addresses in the range 0 to 63. There are \(2^6 = 64\) addresses.

Determine Address Range for Prefix '010'

The prefix '010' fixes the first three bits. Therefore, the range is from `01000000` to `01011111`. This covers host addresses in the range 64 to 95. There are \(2^5 = 32\) addresses.

Determine Address Range for Prefix '011'

The prefix '011' specifies that the first three bits are '011'. Therefore, the range is from `01100000` to `01111111`. This covers host addresses from 96 to 127. There are \(2^5 = 32\) addresses.

Determine Address Range for Prefix '10'

For the '10' prefix, the first two bits are '10', and the range is from `10000000` to `10111111`. This covers host addresses from 128 to 191. There are \(2^6 = 64\) addresses.

Determine Address Range for Prefix '11'

The prefix '11' means the first two bits are '11'. The range is from `11000000` to `11111111`. This covers host addresses from 192 to 255. There are \(2^6 = 64\) addresses.

Key concepts

Longest Prefix Matching

In the world of data networking, routers need to decide the best path for forwarding packets efficiently. Longest prefix matching is a technique used by routers to determine the next destination. It involves selecting the longest continuous matching bits in a destination address from the forwarding table. This helps in making sure packets get routed accurately without confusion or congested paths.
When a packet arrives at the router, its destination address is checked against the router’s forwarding table. The entry with the longest matching prefix from the table is selected for forwarding. This ensures that packets take the most precise path, often referred to as the most specific route, to their destination.
By utilizing longest prefix matching, networks optimize routing decisions, which helps in controlling traffic and enhancing performance.

Forwarding Table

A forwarding table is like the router’s mapbook, containing instructions on where to send incoming packets. It lists destination prefixes and their associated router interfaces, guiding packets to the next hop.
The forwarding table in question uses prefixes such as '00', '010', '011', '10', and '11' to specify different paths. Each entry tells the router which interface will handle packets destined for addresses with a matching prefix.

• The prefix '00' relates to interface 0.
• Prefix '010' directs packets to interface 1.
• '011' uses interface 2.
• Both '10' and '11' connect to interface 2 and interface 3, respectively.

Having an organized forwarding table enables efficient data routing across various networks, reducing latency and avoiding unnecessary detours.

Router Interfaces

Router interfaces are the hands of the router, connecting it to different network segments, and through which all data flows in and out. Each interface is linked to either a point-to-point connection or a network segment.
In the example, the router uses four interfaces labeled 0, 1, 2, and 3, each associated with specific prefix matches in the forwarding table. Every interface directs traffic based on these prefix rules.

• Interface 0 manages traffic for addresses starting with '00'.
• Interface 1 is responsible for '010' prefixed addresses.
• Interface 2 handles two ranges: those starting with '011' and '10'.
• Interface 3 is for addresses with a '11' prefix.

By segregating traffic efficiently through different interfaces, routers can manage network congestion, optimize path selection, and provide reliable connectivity.

Host Address Range

The host address range specifies the potential addresses within a specific network region that can be reached via the router. It's determined by the number of bits remaining after prefix bits have been defined, allowing calculation of all possible addresses.
For example, a prefix like '00' implies that the first two bits of the address are fixed, with the remaining six bits free to vary. This results in a total of 64 possible addresses, ranging from 0 to 63, inclusive.
Let's break down each range given the prefixes:

• For '00', addresses range from 0 (binary 00000000) to 63 (00111111).
• '010' stretches from 64 (01000000) to 95 (01011111), covering 32 addresses.
• '011' covers 96 (01100000) to 127 (01111111), also 32 addresses.
• The prefix '10' spans addresses 128 (10000000) to 191 (10111111), offering 64 addresses.
• '11' gives the largest range from 192 (11000000) to 255 (11111111), with 64 addresses.

Calculating and understanding host address ranges is crucial for network management, ensuring that data packets are sent to the correct devices within a network.