การแบ่งเครือข่ายย่อยและ CIDR เชิงลึก

Subnetting splits a large network into smaller, manageable segments. It improves security, performance, and address efficiency.

CIDR (Classless Inter-Domain Routing) replaced rigid class A/B/C boundaries with flexible prefix lengths.

A CIDR block is written as address/prefix. The prefix counts the number of leading network bits.

• 192.168.1.0/24 — 24 network bits, 8 host bits
• 10.0.0.0/8 — 8 network bits, 24 host bits

A prefix maps directly to a dotted-decimal mask.

• /24 = 255.255.255.0
• /26 = 255.255.255.192
• /30 = 255.255.255.252

Each set bit in the mask is a network bit.

Host count = 2^(host bits) minus 2 (network and broadcast addresses).

• /24 -> 2^8 - 2 = 254 hosts
• /26 -> 2^6 - 2 = 62 hosts
• /30 -> 2^2 - 2 = 2 hosts (perfect for point-to-point links)

Bitwise-AND the IP with the mask to get the network address. The example below computes it in Python.

For 192.168.5.130/26: block size is 64, so subnets start at .0, .64, .128, .192.

The .130 address falls in the 192.168.5.128/26 subnet (usable .129 to .190, broadcast .191).

VLSM lets different subnets use different prefix lengths from one block, matching size to need.

Allocate largest subnets first to avoid fragmentation and wasted address space.

Adjacent subnets can be summarized into one larger prefix, shrinking routing tables.

• 192.168.0.0/24 + 192.168.1.0/24 -> 192.168.0.0/23

Aggregation only works when blocks are contiguous and aligned.

The ipaddress module can split a network into smaller subnets automatically.

Verify whether an address belongs to a network — essential for ACLs and routing decisions.

Watch out for:

• Forgetting to subtract network and broadcast addresses
• Misaligned aggregation that overlaps other blocks
• Using /31 links (valid per RFC 3021 for point-to-point) without device support

Test your subnetting math.

You can now plan address space confidently:

• Read and convert CIDR prefixes and masks
• Calculate network, broadcast, and host ranges
• Apply VLSM to size subnets to need
• Aggregate contiguous blocks to shrink routing tables
• Use Python's ipaddress module for verification

This underpins the IPv6, routing, and NAT topics in this course.