Problem 16 - Computer Networking: A Top-Down Approach

Question

Consider a subnet with prefix \(128.119 .40 .128 / 26\). Give an example of one IP address (of form \(x x x \cdot x x x \cdot x x x \cdot x x x\) ) that can be assigned to this network. Suppose an ISP owns the block of addresses of the form \(128.119 .40 .64 / 26\). Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. What are the prefixes (of form a.b.c. \(\mathrm{d} / \mathrm{x}\) ) for the four subnets?

Step-by-Step Solution

Answer

128.119.40.130 is a valid address for the first subnet. The new subnets are 128.119.40.64/28, 128.119.40.80/28, 128.119.40.96/28, and 128.119.40.112/28.

Step 1: Understanding IPv4 Subnetting

IPv4 addresses consist of 32 bits, divided into four 8-bit octets. A prefix, such as /26, indicates that the first 26 bits are the network part and the remaining bits are for host addresses. A /26 prefix thus implies that the subnet has 64 addresses (2^(32-26)=64).

Step 2: Determine Address Range for 128.119.40.128/26

The subnet 128.119.40.128/26 has a network range from 128.119.40.128 to 128.119.40.191. These addresses include both the network and broadcast addresses, so the usable range is from 128.119.40.129 to 128.119.40.190.

Step 3: Provide an IP Address for 128.119.40.128/26 Network

An example of an IP address that can be assigned in this subnet is 128.119.40.130, as it fits within the usable range of the subnet.

Step 4: Block Information for ISP's 128.119.40.64/26

The ISP owns a /26 block from 128.119.40.64 to 128.119.40.127. Like any /26, it consists of 64 addresses.

Step 5: Divide ISP Subnet into Four Smaller Subnets

To divide the block into four equal subnets, each subnet must have 16 addresses (64 total addresses / 4 subnets = 16 addresses per subnet). A /28 prefix denotes a subnet with 16 addresses (2^(32-28) = 16).

Step 6: Calculate New Subnet Prefixes

The four /28 subnets from the 128.119.40.64/26 block are: 1. 128.119.40.64/28 (range 128.119.40.64 - 128.119.40.79) 2. 128.119.40.80/28 (range 128.119.40.80 - 128.119.40.95) 3. 128.119.40.96/28 (range 128.119.40.96 - 128.119.40.111) 4. 128.119.40.112/28 (range 128.119.40.112 - 128.119.40.127)

Key Concepts

IP Address Range

When dealing with subnetting in IPv4, understanding the range of IP addresses available is crucial. A subnet defines a specific segment of IP address space, determined by its prefix. For example, when considering the subnet 128.119.40.128/26, we acknowledge that this subnet uses the first 26 bits of the address for the network portion. The remaining bits are used for host addresses within the subnet.

The total number of addresses within a /26 prefix is 64, as calculated by the formula: \[2^{(32-26)} = 64\].This range comprises the network address, usable host addresses, and the broadcast address.
In our example, the network address is 128.119.40.128, and the broadcast address is 128.119.40.191. Therefore, the usable IP address range that can be assigned to devices is from 128.119.40.129 to 128.119.40.190.

Subnet Prefix

A subnet prefix helps define the size of a network or subnet. It tells us how many bits are used for the network and how many are left for host addresses. When we see the notation /26 in a subnet prefix,it indicates that the first 26 bits of the 32-bit IP address are designated for the network.

This concept is vital because it helps network engineers understand how many IP addresses are available in a given subnet.Often, an exercise will require creating multiple smaller subnets from a larger one. For example, in the exercise, the ISP wants to divide the 128.119.40.64/26 subnet into four smaller networks. Each smaller subnet needs a /28 prefix because \[2^{(32-28)} = 16\].Thus, each of these smaller subnets will have 16 addresses, allowing for efficient utilization of the given IP space.

Network and Broadcast Address

Network and broadcast addresses frame the usable range in any subnet. The network address is the first address of the subnet and it identifies the subnet itself. For example, in the subnet 128.119.40.128/26, the network address is 128.119.40.128. On the other hand, the broadcast address is the last address of the subnet and it is used to send data to all computers in the subnet.

Continuing with our example, the broadcast address for the subnet 128.119.40.128/26 is 128.119.40.191. Knowing the network and broadcast addresses enables one to clearly delineate the range within which active IP addresses reside and prevents any addressing ambiguities.

ISP Subnet Division

An Internet Service Provider (ISP) may often need to subdivide its allocated address space into smaller sub-networks to manage and distribute IP addresses effectively.

In the scenario given, the ISP owns a block of addresses 128.119.40.64/26 and wants to create four equal subnets. By dividing the 64 addresses within /26 into four groups, each new subnet must have a total of 16 addresses.This is achieved using a subnet prefix of /28 (since \[2^{(32-28)} = 16\]),which results in ranges as follows:- 128.119.40.64/28- 128.119.40.80/28- 128.119.40.96/28- 128.119.40.112/28Planning ISP subnet divisions effectively ensures optimal utilization of IP space and prevents future shortages or conflicts.