IP Subnetting, Troubleshooting IP, and NAT

Why Subnet?

Subnetting divides a larger IPv4 network into smaller logical networks. Each boundary controls broadcasts, organizes addressing, and creates a clearer routing and security design.

Traffic control

Smaller broadcasts

Fewer hosts process each broadcast and local traffic stays within a smaller domain.

Operations

Simpler management

Address ranges can identify buildings, VLANs, departments, and security zones.

Containment

Fault isolation

Failures and policy boundaries can be limited to a smaller segment.

Capacity

Efficient addressing

CIDR and VLSM allocate address space according to actual host needs.

Subnet Terms & Formulas

Subnet count

2ˢ

s is the number of borrowed subnet bits. Modern subnetting includes subnet zero and the all-ones subnet.

Addresses

2ʰ

h is the number of remaining host bits.

Usable hosts

2ʰ − 2

Traditional LAN calculations subtract the network and broadcast addresses.

Block size

256 − mask

Subtract the interesting mask octet from 256 to find subnet increments.

High priorityFind the interesting octet and block size first. The next subnet boundary reveals both the broadcast and valid-host range.

CIDR & Mask Explorer

Select a prefix to see its mask, address capacity, traditional usable hosts, and block size.

Memory stringPartial-octet masks rise 128, 192, 224, 240, 248, 252, 254. Block sizes fall 128, 64, 32, 16, 8, 4, 2.

Interactive Subnet Calculator

Enter an IPv4 address and prefix from /8 through /30.

The Six-Step Method

Begin with the IP address and prefix.

Worked Examples

Fast checkIf the given IP equals the network or broadcast endpoint, it is not a valid traditional host address.

Host Requirement Shortcut

Needed hosts	Host bits	Prefix	Usable
2	2	/30	2
6	3	/29	6
14	4	/28	14
30	5	/27	30
62	6	/26	62
126	7	/25	126
254	8	/24	254

Exam shortcutFind the smallest power of 2 that is at least hosts + 2. Its exponent is h; the prefix is 32 − h.

VLSM Planner

Allocate the largest requirement first, use aligned contiguous boundaries, and record every network, host range, and broadcast before assigning the next block.

Need	Prefix	Network	Valid hosts	Broadcast

Common Subnetting Traps

Four Diagnostic Pings

Run tests from the smallest proven scope outward: stack → interface → local network → remote network.

Start with loopback.

Order mattersA failed ping does not alone prove the target is down; ICMP may be filtered.

Symptom-to-Cause Explorer

NAT & PAT

Fixed mapping

Static NAT

One private address maps to one public address; useful for publishing a server.

Public pool

Dynamic NAT

Private clients receive temporary one-to-one translations from a public pool.

NAT overload

PAT

Many private sessions share one or a few public IPs and are distinguished by ports.

Security noteNAT hides internal address structure but is not a substitute for a firewall.

NAT Address Names & PAT Flow

Name	Meaning	Example
Inside local	Inside host before translation	192.168.1.10
Inside global	Public address representing inside host	203.0.113.20
Outside global	Outside host on public network	198.51.100.40
Outside local	Outside host as represented internally	Often same as outside global

INSIDE LOCAL192.168.1.10:51500

→

PAT TABLERewrite source

→

INSIDE GLOBAL203.0.113.5:40001

→

SERVER198.51.100.20:443

Memory aidInside/outside describes host location. Local/global describes where the address is meaningful.

PowerShell 7 Practice

Get-NetIPConfigurationAddress, gateway, DNS, interface

Get-NetIPAddress -AddressFamily IPv4IPv4 addresses and prefixes

Get-NetRoute -AddressFamily IPv4Connected and default routes

Get-NetNeighbor -AddressFamily IPv4ARP state

Test-Connection 127.0.0.1 -Count 2Local stack

Test-NetConnection 1.1.1.1Remote IP reachability

Resolve-DnsName example.comDNS separately

route print -4IPv4 routing table

Exam Check

Fifteen questions drawn directly from the Chapter 8 manual.

Score: 0 / 0