Fixed Length Subnet Masking
The basic subnetting technique known as Fixed-Length Subnet Masking (FLSM) ensures that each subnet that is formed has the same amount of IP addresses and hosts. Throughout the network, all of these subnets use the same subnet mask, which is fixed. Traditional subnetting or classful subnetting are other names for Fixed Length Subnet Masking.
How Fixed Length Subnet Masking Works
Fixed Length Subnet Masking creates a new subnet section of an IP address by taking a certain amount of bits from the host portion. The number of possible hosts inside each subnet and the total number of subnets that can be formed are both determined by this number of borrowed bits. After a packet with its network number reaches the main gateway of an organization, it is routed to its destination using a subnet number. The Fixed Length Subnet Masking assists the router in determining the pertinent portions of the subnet number.
FLSM Characteristics and Examples
- Same Subnet Size: The number of hosts in each subnet will be the same. For example, each subnet of a network 192.168.1.0/24 with a /26 mask (255.255.255.192) will contain 64 addresses (62 valid hosts). These would be the subnets: 192.168.1.0-63, 192.168.1.128-191, 192.168.1.192-255, and 192.168.64-127.
- Mask Consistency: For every subnet, the subnet mask is the same.
- Example Bit Borrowing: 3 bits taken from the host section of a Class C network (e.g., 255.255.255.0) results in 2^3 = 8 subnets. After that, each subnet would have 2^(8-3) – 2 = 30 valid host addresses, and 255.255.255.224 would be the new subnet mask.
- Another illustration would be splitting an IPv4 prefix, such as 100.100.0.0/24, into two subnets. One bit is taken from the 8-bit host section to make the network part 25 bits. This gives the two new subnets, 100.100.0.0/25 and 100.100.0.128/25, a /25 subnet mask.
Advantages of FLSM
Simplicity: All of the subnets utilize the same mask, making it simple to design, calculate, and configure.
Predictability: Network managers are aware of the precise number of hosts and subnets that are available.
Consistency: Simplifies network configuration in particular contexts by guaranteeing consistent subnet sizes.
Management: Beneficial for networks in which the number of hosts needed by each department or segment is constant.
Router Efficiency: Allows the router to focus on the digits chosen by the mask, saving it the trouble of analyzing a whole IP address.
Disadvantages of FLSM
IP Address Wastage: This is its biggest flaw. Subnets that have different host requirements may squander a lot of IP addresses because smaller subnets are still given a set, larger block of addresses. For instance, a subnet that is intended to accommodate 30 hosts but only requires 10 will waste 20 addresses.
Less Flexible/Efficient: It is not a good fit for networks with widely fluctuating or changing host needs. Variable Length Subnet Masking (VLSM), on the other hand, is more effective at preserving IP addresses and permits a range of subnet sizes.
Implementation of Fixed Length Subnet Masking in Cisco Packet Tracer
Static routes are created once PCs (hosts) and routers are configured with the predefined fixed IP addresses and subnet masks in order to implement Fixed Length Subnet Masking in Cisco Packet Tracer.
Steps to Configure and Verify Fixed Length Subnet Masking:
Select Devices: Launch Cisco Packet Tracer and pick the required devices, including routers, switches, and PCs. You might utilize three PCs, three PT-Switches, and three PT-Routers as an example configuration.
Create Network Topology: Connect the devices with the proper cables, such as automatic connecting cables, after arranging them.
Configure PCs (Hosts): Assign each PC a set subnet mask and an IPv4 address based on a pre-established IP addressing table. Either the desktop IP configuration interface or the command terminal’s ipconfig command can be used to accomplish this.
- Example: Assign 192.168.10.2 to PC0, using 192.168.10.1 as the default gateway and 255.255.255.242 as the subnet mask.
Configure Routers: Apply IPv4 addresses and a uniform subnet mask to each router’s interfaces (such as FastEthernet and Serial ports) in accordance with a router IP addressing table.
- Example: With a subnet mask of 255.255.255.224, router0’s FastEthernet0/0 could be 192.168.10.1, and its Serial 2/0 could be 192.168.10.97.
Assign Static Routes: Establish static routes on each router to allow communication between various subnets once all devices have been configured. In the Command Line Interface (CLI), this is usually accomplished using the command ip route network id> subnet mask> next hop>.
- Example for Router0:
ip route 192.168.10.32 255.255.255.224 192.168.10.98ip route 192.168.10.128 255.255.255.224 192.168.10.98ip route 192.168.10.64 255.255.255.224 192.168.10.98
Verify the Network: The ping command can be used from the command prompt on a PC to check if other network nodes are connected. Pinging successfully shows that the connection is operational.
You can also read Frame Switching: Transfer Data Frames Between Devices