Question

Write a function to do forwarding in an IP router. The procedure has one parameter, an IP address. It also has access to a global table consisting of an array of triples. Each triple contains three integers: an IP address, a subnet mask, and the outline line to use. The function looks up the IP address in the table using CIDR and returns the line to use as its value.

Short answer

Write a function that performs bitwise comparison using CIDR to match the IP against table entries and returns the corresponding output line.

Step-by-step solution

Understand the Problem

We need to write a function that accepts an IP address and determines the correct line in a router by using a lookup table of IP addresses, subnet masks, and output lines. The function should use CIDR (Classless Inter-Domain Routing) to find the correct output line.

Parse the Input Parameters

The function will receive an IP address as a string. We'll also need to interface with a global table which contains multiple triples of (IP address, subnet mask, output line).

Process the Table Entries

Convert each IP address and subnet mask in the table to binary form. This will allow for straightforward comparison to the target IP address.

Implement CIDR Matching

Perform bitwise 'AND' operation between the given IP address and each subnet mask to determine if the resulting prefix matches the prefix of the table entry's IP address. A match will tell us which output line to choose.

Return the Output Line

Once a matching entry is found using CIDR, return the associated output line. If multiple entries match, traditionally the most specific match (largest subnet mask) is preferred.

Key concepts

CIDR Notation

CIDR, or Classless Inter-Domain Routing, is a method used to streamline and manage IP address assignments. Instead of the older classful network design, CIDR allows for variable-length subnet masking. This means that it can allocate IP addresses more efficiently. In CIDR notation, an IP address is followed by a slash and a number which represents the number of bits used for the network address.

For example, in the address "192.168.1.0/24", the "/24" indicates that the first 24 bits are the network part, leaving 8 bits for host addresses within that network. This kind of flexibility helps in reducing the wastage of IP addresses, and it makes routing more efficient by keeping routing tables smaller.

• CIDR is widely used in modern networks as it simplifies the routing process.
• With CIDR, networks can be split into different sizes, fitting the specific needs for various organizations.
• Networks using CIDR notation typically look like "192.168.0.0/22" or similar.

Overall, CIDR notation allows routers to understand and direct the traffic efficiently along the most optimal paths.

Subnet Mask

A subnet mask is used to divide an IP address into a network and host portion. It resembles an IP address itself but serves as a partitioning tool. Subnet masks are significant in determining which part of an IP address refers to the network and which part refers to the hosts:

• A typical subnet mask looks like "255.255.255.0" and specifies the network and host portions of an IP address.
• The subnet mask uses 255 to highlight all bits to use for the network part of the address.

To find the network address from an IP address and subnet mask, a bitwise AND operation is performed. For example:

If the IP is "192.168.1.10" and subnet mask is "255.255.255.0", the network address will be "192.168.1.0".

Understanding and calculating with subnet masks is crucial for networking professionals because it allows them to properly plan address allocations and avoid potential network conflicts.

Router Table Lookup

Router table lookup is a fundamental operation in network routing. A router maintains a routing table that contains paths to various network destinations. When a packet arrives, the router performs a lookup to determine the best path. This process involves the following steps:

• The router examines the destination IP address of the incoming packet.
• It compares this IP against entries in its routing table using CIDR notation and subnet masks.

An efficient router table lookup relies on matching the incoming packet's destination IP with the most specific entry from the table. The best match is usually determined by the subnet with the most bits in the network address.

This process is critical for network performance and is optimized to handle numerous lookups quickly. A successful lookup ensures that data reaches its correct destination as quickly and efficiently as possible. In essence, router table lookup is the crux of directing data traffic across a network.