Classless Inter-Domain Routing (CIDR) is a method used to allocate and manage IP addresses more efficiently by allowing variable-length subnet masks. In CIDR, the traditional IP address classes (Class A, B, and C) are not strictly adhered to, and instead, a prefix length is used to specify the number of network bits in the address.
Let's take an example to illustrate CIDR and classless addressing:
Suppose we have a block of IP addresses assigned to us, let's say 192.168.0.0/16. In this case, the "/16" represents the prefix length, indicating that the first 16 bits of the address represent the network portion.
With the classful addressing system, 192.168.0.0 would be a Class C network, and the remaining 8 bits (32 - 24) would be available for host addresses. This would give us a maximum of 2^8 - 2 = 254 usable host addresses.
However, with CIDR and classless addressing, we can further divide this block into smaller subnets with different prefix lengths to suit our requirements. For instance, we can divide the block into two equal subnets:
- Subnet 1: 192.168.0.0/17 This subnet will have a prefix length of 17, meaning the first 17 bits are the network portion. The remaining 15 bits (32 - 17) will be available for host addresses. This gives us a maximum of 2^15 - 2 = 32,766 usable host addresses.
- Subnet 2: 192.168.128.0/17 Similarly, this subnet will also have a prefix length of 17, with the first 17 bits representing the network portion. The remaining 15 bits can be used for host addresses, giving us another 32,766 usable host addresses.
By using classless addressing, we have effectively split the original Class C network into two smaller subnets, allowing for more efficient allocation of IP addresses based on specific requirements.
CIDR and classless addressing provide greater flexibility in IP address allocation and help conserve IP address space by allowing variable-length subnet masks to create smaller subnets. This is particularly useful in scenarios where organizations need to optimize their address space usage or when assigning addresses to different network segments with varying numbers of hosts.
Let's work with the following example:
Classless IP: 192.168.10.50/26
To get started, we'll first convert the subnet mask from CIDR notation to its binary form:
1. Subnet Mask (CIDR: /26)
Binary Subnet Mask: 11111111.11111111.11111111.11000000
2. Wildcard Mask:
The wildcard mask is the inverse of the subnet mask. To get the wildcard mask, we flip all the bits of the subnet mask.
Binary Wildcard Mask: 00000000.00000000.00000000.00111111
3. Broadcast Address:
To find the broadcast address, we apply the wildcard mask to the host portion of the IP address and set all the host bits to 1.
Binary IP Address: 11000000.10101000.00001010.00110010
Binary Wildcard Mask: 00000000.00000000.00000000.00111111
Calculated Binary Broadcast Address: 11000000.10101000.00001010.00111111
4. Network ID:
To find the network ID, we apply the subnet mask to the IP address.
Binary IP Address: 11000000.10101000.00001010.00110010
Binary Subnet Mask: 11111111.11111111.11111111.11000000
Calculated Binary Network ID: 11000000.10101000.00001010.00110000
5. Host ID:
The remaining bits after the network ID in the IP address represent the host ID.
Binary Host ID: 00000000.00000000.00000000.00011010
Now, let's summarize the information:
- Binary Subnet Mask: 11111111.11111111.11111111.11000000
- Binary Wildcard Mask: 00000000.00000000.00000000.00111111
- Binary Broadcast Address: 11000000.10101000.00001010.00111111
- Binary Network ID: 11000000.10101000.00001010.00110000
- Binary Host ID: 00000000.00000000.00000000.00011010
Route Summary: Since this is a single IP address with a subnet mask of /26, there is no need for route summarization in this case.
VLSM (Variable Length Subnet Masking): VLSM is a technique that allows you to allocate different subnets with different subnet mask lengths within a larger network. It's commonly used to optimize IP address utilization. However, in this example, we only have one subnet, so VLSM isn't required here.
I'll explain VLSM (Variable Length Subnet Masking):
VLSM is a technique used to subnet a network in a way that allows for more efficient utilization of IP address space. It is based on considering the common bits in the Network ID of different subnets. By doing this, route summarization can be performed as long as there are common bits in the Network ID, and the remaining bits are used for the User ID.
Let's break down the explanation:
1. Subnetting:
When you have a large network and need to divide it into smaller subnetworks (subnets), subnetting is the process of creating these smaller divisions. Each subnet is allocated a range of IP addresses that it can use for its devices.
2. Network ID and User ID:
In an IP address, the Network ID represents the portion of the address that identifies the specific subnet to which the device belongs. It's common among all the devices within that subnet. On the other hand, the User ID (also known as Host ID) is the portion of the IP address that uniquely identifies each individual device within the subnet.
3. Common Bits in Network ID:
In VLSM, when you create subnets, you identify common bits in the Network ID across those subnets. These common bits indicate the higher-level network that encompasses all the subnets. By identifying these common bits, you can perform route summarization, which means you can represent multiple subnets with a single, larger network address. This helps reduce the size of routing tables and makes routing more efficient.
4. Route Summarization:
Route summarization, also known as route aggregation, is a process where multiple smaller network addresses are represented by a larger network address. This larger address covers all the individual smaller addresses. By summarizing routes, routing devices (such as routers) can use a single entry in their routing table for multiple subnets, simplifying and speeding up the routing process.
5. Remaining Bits for User ID:
After summarizing the common bits in the Network ID for route summarization, the remaining bits in the Network ID are unique to each subnet. These unique bits are then used as the User ID, allowing each subnet to assign unique IP addresses to its devices.
In summary, VLSM allows you to divide a larger network into smaller subnets while efficiently using IP address space. By identifying common bits in the Network ID, you can perform route summarization, and the remaining bits are used to uniquely identify devices within each subnet. This helps optimize network addressing and routing.