Fixed-length subnet masking (FLSM) divides an IPv4 address block into equal-sized subnets. Borrowing host bits creates more subnets while reducing addresses available in each subnet. FLSM is useful for learning subnetting and remains relevant to CCNA exam practice, although VLSM uses addresses more efficiently. section 11.2 section 11.5
Use this process to solve an FLSM subnetting problem:
- Borrow enough host bits to create the required number of subnets; each borrowed bit doubles the subnet count. section 11.2.4
- Add the borrowed bits to the original prefix length to obtain the new prefix. section 11.2.4
- Calculate usable hosts with 2^y − 2, where y is the remaining number of host bits. section 11.2.4
- Identify network, broadcast, first usable, and last usable addresses for every subnet. section 11.5
172.25.190.0/23 divided into four subnets
Borrow 2 bits, producing four /25 subnets with 126 usable hosts each: 172.25.190.0/25, 172.25.190.128/25, 172.25.191.0/25, and 172.25.191.128/25. The first usable addresses are 172.25.190.1, 172.25.190.129, 172.25.191.1, and 172.25.191.129. section 11.2.4
