Question

A router has the following (CIDR) entries in its routing table: \(\begin{array}{ll}\text { Address/mask } & \text { Next hop } \\ 135.46 .56 .0 / 22 & \text { Interface } 0 \\ 135.46 .60 .0 / 22 & \text { Interface } 1 \\\ 192.53 .40 .0 / 23 & \text { Router } 1 \\ \text { default } & \text { Router } 2\end{array}\) For each of the following IP addresses, what does the router do if a packet with that address arrives? (a) \(135.46 .63 .10\) (b) \(135.46 .57 .14\) (c) \(135.46 .52 .2\) (d) \(192.53 .40 .7\) (e) \(192.53 .56 .7\)

Short answer

(a) Interface 1; (b) Interface 0; (c) Router 2; (d) Router 1; (e) Router 2.

Step-by-step solution

CIDR and Subnet Mask Recap

Each CIDR entry has two parts: an IP address and a subnet mask. This determines the network portion of an IP address. For example, in 135.46.56.0/22, the subnet mask is 255.255.252.0 because 22 bits are used for the network portion.

Determine Network Address for Each Entry

Calculate the network address by applying the subnet mask to both the routing table entries and the target IP addresses. For instance, for 135.46.56.0/22, the network address is obtained by performing a bitwise AND operation with the subnet mask.

Match IP Address Against Routing Table

Compare each given IP address to the network address of the routing table entries. A matching network address indicates the route for the IP.

Process IP Address (a) 135.46.63.10

Apply the /22 mask from both 135.46.56.0/22 and 135.46.60.0/22. The network address for 135.46.60.0/22 covers 135.46.60.0 to 135.46.63.255. 135.46.63.10 is within this range, so it is routed to Interface 1.

Process IP Address (b) 135.46.57.14

Apply the /22 mask to 135.46.56.0/22. The network range is from 135.46.56.0 to 135.46.59.255. Here, 135.46.57.14 falls within this range, so it is routed to Interface 0.

Process IP Address (c) 135.46.52.2

The /22 mask for 135.46.56.0/22 does not cover 135.46.52.2, nor does 135.46.60.0/22. The default route is then used, and the packet is routed to Router 2.

Process IP Address (d) 192.53.40.7

Using the /23 mask, the network address for 192.53.40.0/23 is from 192.53.40.0 to 192.53.41.255. The given address, 192.53.40.7, falls within this range, so it is routed to Router 1.

Process IP Address (e) 192.53.56.7

This address doesn't match any of the explicit entries (since neither 135.46.x.x nor 192.53.40.0/23 include it). Therefore, use the default route. It is sent to Router 2.

Key concepts

Routing Tables

Routing tables are essential components in networking devices like routers. They serve as the roadmap for data packets to reach their destination on the internet or within a network. A routing table contains rules that guide where data packets should go based on the destination IP address.
Each entry in a routing table consists of:

• A network destination address
• A subnet mask
• A next hop or interface

The routing table entries use CIDR notation, which combines an IP address with a subnet mask, like 135.46.56.0/22. This helps the router determine the most efficient path for forwarding packets.
When a packet arrives, the router checks its routing table to find the best match for the packet's destination address. The router identifies the correct path by performing a bitwise AND operation between the subnet mask and the IP address. If the network address derived from this operation matches one of the entries, the packet follows that route.
This step-by-step process ensures that data travels correctly across various interconnected networks, optimizing both speed and security.

Subnet Masks

Subnet masks are critical elements in the networking world, used to determine which part of an IP address represents the network and which part denotes the host within that network.
A subnet mask is typically written in terms of bits, like /22, which signifies the first 22 bits are for the network portion. This is translated into an expanded form such as 255.255.252.0, which a router uses to interpret the IP addresses.
When a network device encounters an incoming IP packet, it uses the subnet mask to isolate the network portion of the destination IP address. This enables the device to decide whether the packet belongs to the same local network or needs to be forwarded to another network.
For example, with the entry 135.46.56.0/22, the first 22 bits of any incoming IP will be compared against this entry's network address derived from the subnet mask. Matching this allows the router to route packets accurately, ensuring efficient data traffic flow.

Network Address

The network address is a fundamental concept in understanding IP networks. It represents the starting point of an IP network; every host within this network falls under its range.
Derived from an IP address and a subnet mask, the network address is calculated with a bitwise AND operation. For example, an IP of 135.46.56.0 combined with a subnet mask of 255.255.252.0 results in a network address of 135.46.56.0. This operation ensures that all IPs in the range from 135.46.56.0 to 135.46.59.255 belong to the same network.
Routers utilize network addresses to direct data packets to the correct destination. When a packet's destination IP matches a network address entry in the routing table, it follows the associated route. This reduces unnecessary data transmission across networks and maintains efficiency. Understanding network addresses is vital for configuring networks and ensuring precise routing.

Default Routes

Default routes are the backup paths in a routing table that catch any IP packet that doesn't match specific entries. Serving as a safety net, these routes ensure that packets still find a way out even if there's no detailed address match.
Think of a default route as a "catch-all" route. If a packet's destination IP doesn't fit any of the routing table's specified networks, it is sent via the default route. This is key for simplifying routers' work because they don't need to account for every possible destination in exhaustive detail.
For instance, if a packet addressed to 192.53.56.7 arrives but doesn't match any routing table specifics, it will default to the designated next hop, often another router, as seen with Router 2 in this example.
Default routes are crucial for network reliability and resource conservation, allowing lesser-known or unexpected IP addresses to still reach their destination effectively by traversing through general paths.