Subnetting

Subnetting - take a large network to split it up into smaller netowkr.s

Subnet masks modify netorks and create subnets.

10.0.0.0/x 16.7

10.0.0.0/8 16.7 million

10.0.0.0/24 256 IPs

10.0.1.0/24 256 ips

10.0.2.0/24 256

A 16.7 MIL, class b 65,5543, class c 254

/25 , /26, /27 etc can increase the networks, and reduce the host’s per nework.

2^s = numer of hosts

you need to use the -2 for the netowrk and the broadcast the first and last in the rnge.

2h -2 h= number of hosts.

CIDR notation.

classless interdomain routing. 192.168.1.0/26

Variable - Legnth subnet mask (VLSM)

Allows subnets of vaious size to be used and requirs a protocol that supports it.

RIP OSPF egrip all support it. it’s a subnet of subnets.

chart ffrom /24 to /30 with ips and number of subnets. will help with test day.

Subnetting (N10-009)

What it is

Breaking a larger network into smaller subnets to improve performance, security, and address use.

Masks & CIDR

Subnet mask splits network bits (1s) and host bits (0s).
CIDR notation: /prefix = number of network bits.
- Example: 10.0.0.0/8 (~16.7M host addresses total), 10.0.0.0/24 (256 addresses), 10.0.1.0/24, 10.0.2.0/24, etc.

Core formulas (memorize)

Usable hosts per subnet: 2^(host bits) − 2
Number of subnets when you borrow s bits (from an original block): 2^s
Block size in the changing octet: 256 − mask_octet
- e.g., mask 255.255.255.192 → 256 − 192 = 64 → subnets start at .0, .64, .128, .192

VLSM (Variable Length Subnet Mask)

Use different prefix lengths in the same major network to fit varied host needs.
Supports VLSM: RIPv2, OSPF, EIGRP, IS-IS, BGP
- Does NOT support: RIPv1 (exam trap)

High-Yield Example

Need about 60 hosts? Choose /26: mask 255.255.255.192, 62 usable.

Subnets of 192.168.1.0/24 split to /26 (block size 64):

192.168.1.0/26 → hosts .1–.62, bcast .63
192.168.1.64/26 → hosts .65–.126, bcast .127
192.168.1.128/26 → hosts .129–.190, bcast .191
192.168.1.192/26 → hosts .193–.254, bcast .255

Quick Subnet Chart (/24 to /30)

| Prefix | Mask | Usable Hosts | Block Size (4th octet) | | --- | --- | --- | --- | | /24 | 255.255.255.0 | 254 | 1 | | /25 | 255.255.255.128 | 126 | 128 | | /26 | 255.255.255.192 | 62 | 64 | | /27 | 255.255.255.224 | 30 | 32 | | /28 | 255.255.255.240 | 14 | 16 | | /29 | 255.255.255.248 | 6 | 8 | | /30 | 255.255.255.252 | 2 | 4 |

Memory trick: block sizes walk down by halves: 128, 64, 32, 16, 8, 4 → /25 → /30.

Must-Know Classful Defaults (legacy but tested)

Class A /8 (255.0.0.0) → ~16.7M addresses
Class B /16 (255.255.0.0) → 65,536 addresses
Class C /24 (255.255.255.0) → 256 addresses

(Usable = total − 2)

Exam Tips (rapid)

“~60 hosts?” → /26. “~30?” → /27. “~14?” → /28.
Network = first address (all host bits 0). Broadcast = last (all host bits 1).
RIPv1 no VLSM; RIPv2/OSPF/EIGRP yes VLSM.
Block size trick saves time on finding ranges.

5 Rapid Review Qs

Usable hosts in /27? → 30
Block size for 255.255.255.224? → 32
First usable in 10.0.5.64/26? → 10.0.5.65
Broadcast of 172.16.10.0/28? → 172.16.10.15
Protocol that doesn’t support VLSM? → RIPv1

Want a printable one-page subnetting cheat sheet with more prefixes (/16–/32) and quick practice?