IP Addressing & Subnetting

This section is one of the highest-yield topics on Network+. You must be able to:

• Identify IPv4 vs IPv6 formats
• Recognize private vs public IP ranges
• Convert between dotted decimal, binary, and CIDR notation
• Calculate subnets, hosts, usable addresses, and subnet masks quickly
• Solve subnetting problems in under 60 seconds per question

IPv4 Addressing Basics

• 32-bit address, written in dotted decimal (four octets, 0–255 each)
  Example: 192.168.10.45
• Classes (legacy, still useful for understanding defaults):
  • Class A: 1.0.0.0 – 126.255.255.255 (/8), 16M hosts
  • Class B: 128.0.0.0 – 191.255.255.255 (/16), 65K hosts
  • Class C: 192.0.0.0 – 223.255.255.255 (/24), 254 hosts
  • Class D: 224.0.0.0 – 239.255.255.255 (multicast)
  • Class E: 240.0.0.0+ (experimental)

Private IPv4 Ranges (RFC 1918) – Never routed on public Internet

• 10.0.0.0 – 10.255.255.255 (/8)
• 172.16.0.0 – 172.31.255.255 (/12)
• 192.168.0.0 – 192.168.255.255 (/16)

Special IPv4 Addresses

• 127.0.0.1 – Loopback (localhost)
• 169.254.0.0/16 – APIPA (Automatic Private IP – when DHCP fails)
• 0.0.0.0 – Default route / any address
• 255.255.255.255 – Limited broadcast

Subnetting & CIDR Notation

CIDR (Classless Inter-Domain Routing) replaces classful addressing.
/ notation = number of network bits (1s in mask).
Subnet mask = 32 bits, contiguous 1s followed by 0s.

Common subnet masks & host counts:

CIDR	Subnet Mask	Binary (last octet example)	Total Addresses	Usable Hosts	Increment (block size)
/24	255.255.255.0	11111111	256	254	1
/25	255.255.255.128	10000000	128	126	128
/26	255.255.255.192	11000000	64	62	64
/27	255.255.255.224	11100000	32	30	32
/28	255.255.255.240	11110000	16	14	16
/29	255.255.255.248	11111000	8	6	8
/30	255.255.255.252	11111100	4	2	4
/31	255.255.255.254	11111110	2	2 (point-to-point)	2
/32	255.255.255.255	11111111	1	1 (host route)	—

Formula cheat sheet:

• Total addresses per subnet = 2^(32 – CIDR suffix)
• Usable hosts = 2^(32 – CIDR) – 2 (subtract network + broadcast)
  Exception: /31 has 2 usable (no network/broadcast waste in point-to-point)

Quick Subnetting Method (Most Common Exam Style)

Given network: 192.168.10.0 /24
Question: Create 6 subnets. How many bits to borrow? What are the subnets?

1. Need at least 6 subnets → 2^3 = 8 subnets (borrow 3 bits)
2. New mask = /24 + 3 = /27
3. Block size (increment) = 256 – 224 = 32 (or 2^(8–3) = 32)
4. Subnets:
   192.168.10.0 /27
   192.168.10.32 /27
   192.168.10.64 /27
   192.168.10.96 /27
   192.168.10.128 /27
   192.168.10.160 /27
   (and two more unused)

For each subnet:

• Network ID = first address
• First usable = network + 1
• Last usable = broadcast – 1
• Broadcast = next subnet – 1

Example for 192.168.10.64 /27:

• Network: 192.168.10.64
• First host: 192.168.10.65
• Last host: 192.168.10.94
• Broadcast: 192.168.10.95

IPv6 Addressing Basics

• 128-bit address, hexadecimal, 8 groups of 4 hex digits
  Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• Shorthand rules:
  • Leading zeros in each group can be omitted
  • One consecutive section of all zeros → double colon (::) once only
    Example: 2001:db8::8a2e:370:7334

IPv6 Address Types

• Global Unicast: 2000::/3 (public, routable)
• Unique Local: fc00::/7 (private, like IPv4 private)
• Link-Local: fe80::/10 (auto-generated on every interface, used for local comms)
• Multicast: ff00::/8
• Loopback: ::1
• Unspecified: ::

IPv6 Subnetting

• Standard LAN size: /64 (huge – 2^64 hosts)
• Common prefixes: /48 or /56 for organizations, then subnet to /64 per LAN
• No NAT needed – global addresses everywhere

Example: 2001:db8🔡0000::/48

• Subnet to /52 → 16 subnets (0000 to f000 in fourth hextet)
• Subnet to /64 → 65,536 subnets per /48

Exam Hot Spots & Practice Questions Types

1. Given IP and mask, identify network ID, broadcast, usable range
2. How many subnets/hosts if borrow X bits?
3. What is the new CIDR after borrowing bits for Y subnets?
4. Convert mask to binary or vice versa
5. Identify private vs public IP
6. IPv6 shorthand / type identification
7. VLSM (variable-length subnet mask) – allocate subnets of different sizes efficiently

Quick Reference Cheat Sheet (Memorize)

• /24 = 256 total, 254 hosts
• Each bit borrowed: subnets ×2, hosts ÷2
• Last octet subnetting: 256 – mask value = increment
• IPv6: /64 = standard LAN, ::1 = loopback, fe80:: = link-local

Study Strategy

1. Practice subnetting 20–30 problems daily (use subnetting.net, ipcalc, or Professor Messer worksheets).
2. Time yourself: aim for less than 45 seconds per basic subnet question.
3. Memorize private ranges, common / notation host counts, and IPv6 prefixes.
4. Flashcards: Front = 192.168.5.0/26 → Back = mask, subnets possible, usable hosts, range
5. Understand why we subnet: reduce broadcast domain, security, efficient IP use.

Master subnetting – it appears in PBQs (calculate subnets, assign addresses) and dozens of multiple-choice questions. Nail this and most of Domain 1 becomes easier.