Question

Suppose \(\mathrm{P}, \mathrm{Q}\), and \(\mathrm{R}\) are network service providers, with respective CIDR address allocations (using the notation of Exercise 45) C1.0.0.0/8, C2.0.0.0/8, and \(C 3.0 .0 .0 / 8\). Each provider's customers initially receive address allocations that are a subset of the provider's. P has the following customers: PA, with allocation C1.A3.0.0/16, and PB, with allocation C1.B0.0.0/12. Q has the following customers: QA, with allocation C2.0A.10.0/20, and \(\mathrm{QB}\), with allocation \(\mathrm{C} 2.0 \mathrm{~B} .0 .0 / 16\). Assume there are no other providers or customers. (a) Give routing tables for \(\mathrm{P}, \mathrm{Q}\), and \(\mathrm{R}\), assuming each provider connects to both of the others. (b) Now assume \(P\) is connected to \(Q\) and \(Q\) is connected to \(R\), but \(P\) and \(R\) are not directly connected. Give tables for \(\mathrm{P}\) and \(\mathrm{R}\). (c) Suppose customer PA acquires a direct link to Q, and QA acquires a direct link to \(\mathrm{P}\), in addition to existing links. Give tables for \(\mathrm{P}\) and \(\mathrm{Q}\), ignoring \(\mathrm{R}\).

Short answer

(a) Routing tables include all providers. (b) P routes through Q to R. (c) P and Q have direct routes for customers with new links.

Step-by-step solution

Understanding Address Allocations

The providers have these address blocks: - P: C1.0.0.0/8 - Q: C2.0.0.0/8 - R: C3.0.0.0/8 The customers have these allocations: - PA: C1.A3.0.0/16 - PB: C1.B0.0.0/12 - QA: C2.0A.10.0/20 - QB: C2.0B.0.0/16

Routing Table for P, Q, and R (Part a)

Each provider connects to both others, so their routing tables will include direct routes to each other's address blocks.- P's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | direct | | C2.0.0.0/8 | Q | | C3.0.0.0/8 | R | | C1.A3.0.0/16| direct | | C1.B0.0.0/12| direct |- Q's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | P | | C2.0.0.0/8 | direct | | C3.0.0.0/8 | R | | C2.0A.10.0/20| direct | | C2.0B.0.0/16| direct |- R's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | P | | C2.0.0.0/8 | Q | | C3.0.0.0/8 | direct |

Routing Tables for P and R (Part b)

For this part, P is only connected to Q, and Q is connected to R.- P's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | direct | | C2.0.0.0/8 | Q | | C3.0.0.0/8 | Q | | C1.A3.0.0/16| direct | | C1.B0.0.0/12| direct | - R's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | Q | | C2.0.0.0/8 | Q | | C3.0.0.0/8 | direct |

Routing Tables for P and Q (Part c)

Now PA has a direct link to Q, and QA has a direct link to P.- P's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | direct | | C2.0.0.0/8 | Q | | C1.A3.0.0/16| direct | | C1.B0.0.0/12| direct | | C2.0A.10.0/20| QA | | C2.0B.0.0/16| Q | - Q's Table: | Network | Next Hop | |-------------|----------| | C1.0.0.0/8 | P | | C2.0.0.0/8 | direct | | C3.0.0.0/8 | R | | C2.0A.10.0/20| direct | | C2.0B.0.0/16| direct | | C1.A3.0.0/16| PA |

Key concepts

Classless Inter-Domain Routing (CIDR)

Classless Inter-Domain Routing, or CIDR, is a method for allocating IP addresses and routing Internet Protocol packets. It replaces the traditional class-based IP address system. Here's how it works:
CIDR allows for the creation of IP addresses more efficiently. Instead of being confined to classes like A, B, or C, CIDR uses a prefix length to determine the size of the network.

• An IP address includes a network part and a host part. In CIDR notation, an IP address is followed by a '/' and a number (e.g., 192.168.1.0/24).
• This number is the prefix length and it indicates how many bits are used for the network part.

For example, in C1.A3.0.0/16, '16' tells us the first 16 bits are network bits, and the rest are for hosts.
This flexibility helps network providers allocate IP addresses without wasting them.

Routing Tables

Routing tables are critical for directing data packets through a network. They are like maps for your data.
Each entry in a routing table has a destination network address, a prefix length, and a next-hop address, which tells where to send packets destined for that network.
Let's break down P's routing table for better understanding with CIDR calculation and routing entries:

• Network: C1.0.0.0/8, Next Hop: direct
• Network: C2.0.0.0/8, Next Hop: Q
• Network: C3.0.0.0/8, Next Hop: R
• Network: C1.A3.0.0/16, Next Hop: direct
• Network: C1.B0.0.0/12, Next Hop: direct

These entries essentially instruct provider P where to send packets based on the destination IP.

Network Providers

Network providers, like P, Q, and R in our example, manage large blocks of IP addresses. They allocate these addresses to their customers and set up routing paths.
Here’s how they are structured:
Provider P manages the network starting from C1.0.0.0/8, Q from C2.0.0.0/8, and R from C3.0.0.0/8. Each has subsets for their specific customers:

• Provider P allocates C1.A3.0.0/16 to PA and C1.B0.0.0/12 to PB.
• Provider Q allocates C2.0A.10.0/20 to QA and C2.0B.0.0/16 to QB.

In some cases, providers may be interconnected, making routing more efficient. For example, P and Q are directly connected, and Q is also connected to R, ensuring data can flow efficiently between these networks.

IP Address Allocation

IP address allocation is how network providers assign IP blocks to their clients. It needs careful planning to ensure efficient use and minimal waste.
Consider the IP blocks discussed:

• Provider P has a broader range, C1.0.0.0/8, and assigns subsets to its customers PA and PB.
• PA gets C1.A3.0.0/16, meaning 16 bits identify the network and the rest for hosts. PB gets C1.B0.0.0/12.

This segmentation, enabled by CIDR, makes allocation flexible and scalable.
It helps to prevent the exhaustion of IP address spaces, while still meeting the needs of clients.

Subnetting

Subnetting is the process of dividing a larger network into smaller, more manageable sub-networks. It’s a crucial concept in CIDR.
Using subnetting, providers can allocate smaller chunks of their address spaces to different customers. For instance:

• Provider P allocates to PA: C1.A3.0.0/16, which could be further divided if needed.
• PB receives a larger CIDR block, C1.B0.0.0/12, potentially indicating more hosts or additional subnets.

Subnetting enhances routing efficiency and improves security by isolating network segments. It allows providers to use IP address space efficiently, reducing unnecessary allocations, and optimizing routing processes.