1.3IP addressing and subnet design

Subnet design is not a "plug into the formula and get the answer" calculation exercise—it is genuine practical judgment work: allocating a limited address space without waste, sized to the required host count of each department or floor. This section builds an understanding of how CIDR/VLSM work, then practices deriving a subnet layout from actual requirements (the number of devices per department) and manually verifying the arithmetic.

1.3.1IPv4 address structure and classes

• An IPv4 address is 32 bits, expressed in decimal as four 8-bit octets (e.g., 192.168.1.10). Historically, the leading bit pattern classified addresses into Class A/B/C (with a fixed 8/16/24-bit network portion respectively), but today CIDR makes a variable-length network portion, unconstrained by class, the mainstream approach.
• CIDR (Classless Inter-Domain Routing) lets you freely specify the number of network-portion bits as a prefix length, such as /24. 192.168.1.0/24 means "the leading 24 bits are the network portion," and it reduces wasted address allocation by not being constrained by class.

1.3.2Subnet masks and VLSM

• A subnet mask is a 32-bit value indicating how much of an IP address is the network portion (255.255.255.0 = /24). Within the host portion, the addresses actually assignable exclude "all bits 0" (the network address) and "all bits 1" (the broadcast address).
• VLSM (Variable Length Subnet Mask) is the technique of splitting a single address block into subnets of different lengths sized to each department's required host count. Unlike fixed-length splitting that assigns the same size to every department, it minimizes wasted addresses.

Suppose that, as a network designer, you are given the requirement to allocate 192.168.10.0/24 (256 addresses) across three departments using VLSM: Sales (100 hosts needed), Development (50 hosts needed), and General Affairs (20 hosts needed). First, since Sales needs to accommodate 100 devices, find the smallest n satisfying 2^n - 2 >= 100 for the host portion: 2^7-2=126 >= 100 (n=7 suffices; n=6 gives only 2^6-2=62, which falls short), so you assign a 7-bit host portion, i.e., a /25 prefix (255.255.255.128). Reserve 192.168.10.0/25 (192.168.10.0 through 192.168.10.127, of which .1 through .126—126 addresses—are assignable) for Sales, and verifying, 126 >= 100 satisfies the requirement. From the remaining 192.168.10.128/25 (128 addresses), carve out 50 hosts for Development. The smallest n satisfying 2^n-2 >= 50 is n=6 (2^6-2=62 >= 50), so assign a /26 prefix (255.255.255.192): 192.168.10.128/26 (.128 through .191, with .129 through .190—62 addresses—assignable). Finally, for General Affairs' 20 hosts, carve further out of the remaining 192.168.10.192/26 (64 addresses); the smallest n satisfying 2^n-2 >= 20 is n=5 (2^5-2=30 >= 20), so assign a /27 prefix (255.255.255.224): 192.168.10.192/27 (.192 through .223, with .193 through .222—30 addresses—assignable), satisfying the requirement while preserving the remaining 192.168.10.224/27 (30 addresses) for future expansion. Compared to a plan that assigns a fixed /26 (62 hosts) to every department, VLSM accommodates Sales' large demand of 100 hosts while minimizing surplus addresses at smaller departments—this is the rational judgment behind segment design.

1.3.3Section summary

• CIDR specifies the network portion via prefix length. Host count = 2^(host bits) - 2 (excluding all-0/all-1)
• VLSM splits subnets into variable lengths sized to each department's required host count, minimizing wasted addresses
• Subnet design should be carried out as a judgment exercise grounded in manual verification plus requirements (host count needed, future expansion)