Subnetting

Computer Network & Network Security System — Network Layer, NEC licence examination syllabus (Nepal Engineering Council).

Subnetting: carving one network into many, on paper

The single most-tested numerical in this entire subject — get comfortable with it, not just familiar.

subnets = 2ⁿ, hosts per subnet = 2^(32 − prefix − n) − 2 With prefix = 24, n = 2, newmask = 26.

Start at /24 with 2 bits borrowed: 4 subnets of 62 usable hosts, which is the worked example above. Borrow a third bit and you get 8 subnets of 30. Two addresses always disappear from each subnet — one for the network itself and one for broadcast.

An organization gets handed one big network (say, a Class C block of 254 usable addresses) but actually has five separate departments that shouldn't all be on one flat network. Subnetting lets them slice that one block into several smaller ones — without asking anyone for more address space.

Worked subnetting example: Given: 192.168.1.0/24, need at least 4 subnets. Step 1: Subnets needed = 4 → need 2^n ≥ 4 → borrow n=2 bits from host part. Step 2: New subnet mask = /24 + 2 = /26 → 255.255.255.192 Step 3: Each subnet size = 2^(remaining host bits) = 2^6 = 64 addresses (62 usable, since 1 is network address, 1 is broadcast) Step 4: The four subnets are: 192.168.1.0 /26 (hosts .1–.62, broadcast .63) 192.168.1.64 /26 (hosts .65–.126, broadcast .127) 192.168.1.128 /26 (hosts .129–.190,broadcast .191) 192.168.1.192 /26 (hosts .193–.254,broadcast .255)

The pattern to remember: borrowing n bits from the host portion always gives you 2ⁿ subnets, each with 2^(remaining host bits) total addresses (usable = that minus 2, for network + broadcast).

🔧 CIDR and VLSM

CIDRClassless Inter-Domain Routing — writes the network portion as a simple /n suffix (like /24), abandoning the rigid class-based boundaries entirely.

SupernettingThe reverse of subnetting — combining several smaller networks into one larger advertised block, to keep routing tables smaller.

VLSMVariable Length Subnet Masking — lets DIFFERENT subnets within the same network use DIFFERENT mask sizes, so a department needing 500 hosts doesn't get the same slice as one needing 10.

💡 Practice subnetting numericals in both directions: "given a network and required number of subnets, find the mask" AND "given a mask, find how many subnets and hosts per subnet are possible." Both directions get asked.

Syllabus points

Subnet mask, default masks
Subnetting numericals (subnets, hosts, ranges)
CIDR / supernetting, VLSM

Create a free account to tick topics off, take notes as you read, watch the video lessons and get a day-by-day study plan built around your exam date.