IP Addressing & Subnetting (Network+ N10-009) | Interactive Study Guide 2026

IP Addressing & Subnetting

The foundation of every network — understanding how addresses are structured, assigned, and divided is Domain 1 of the N10-009 exam (23% of total score).

🌍

IPv4 Fundamentals

32-Bit Addressing

IPv4 uses 32-bit addresses written in dotted-decimal notation. Five address classes define default network sizes, with three private ranges reserved for internal use.

Format: 4 octets (e.g., 192.168.1.1) Classes: A, B, C, D (multicast), E (experimental) Private ranges: 10/8 · 172.16/12 · 192.168/16 APIPA: 169.254.0.0/16 (auto-assigned, DHCP fail) Loopback: 127.0.0.1 (tests local TCP/IP stack)

✂️

Subnetting & CIDR

Dividing Networks

CIDR (Classless Inter-Domain Routing) notation specifies how many bits are the network prefix. Subnetting divides a larger network into smaller, manageable segments.

CIDR: /prefix = number of network bits Mask: /24 = 255.255.255.0 Hosts: 2^h − 2 (h = host bits) Block size: 256 − last subnet mask octet /30: Minimum — 2 hosts, WAN links

🔢

IPv6

128-Bit Addressing

IPv6 uses 128-bit addresses in colon-separated hexadecimal. It eliminates broadcast, uses multicast and anycast, and includes SLAAC for stateless auto-configuration.

Format: 8 groups of 4 hex digits Global unicast: 2000::/3 (internet-routable) Link-local: fe80::/10 (auto-configured, non-routable) Loopback: ::1/128 No broadcast — multicast replaces it (ff00::/8)

⚙️

Address Services

DHCP, NAT & More

Address services automate IP assignment (DHCP), translate private to public addresses (NAT/PAT), and provide the glue that connects private networks to the internet.

DHCP DORA: Discover → Offer → Request → Ack Static NAT: 1-to-1 private↔public mapping PAT/Overload: Many-to-1 using port numbers Default gateway: Router IP on local subnet DNS: Resolves names to IPs (port 53)

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Exam focus: Domain 1 (Networking Fundamentals) is 23% of the N10-009 exam. Subnetting calculation questions — finding network addresses, broadcast addresses, and usable host counts from CIDR notation — appear on nearly every attempt. Practice until the /24–/30 table is instant recall.

Why IP Addressing Matters

Every device on a network needs a unique IP address to send and receive data. The subnet mask determines which part of the address identifies the network and which part identifies the host. Getting this wrong causes misrouting, connectivity failures, and security gaps — all common real-world troubleshooting scenarios the exam tests.

23%

of N10-009 exam

4.3B

total IPv4 addresses

340 undecillion

IPv6 address space

How It Works

From binary bits to usable subnets — the mechanics of IP addressing explained step by step.

IPv4 Binary Structure — 192.168.1.130/25

Decimal: 192 . 168 . 1 . 130

Binary:

1

0

.

1

0

1

0

1

0

.

0

1

.

1

0

1

0

←————————— /25 Network Portion (25 bits) —————————→ ←— Host (7 bits) —→

Network bits (fixed — identify the subnet)

Host bits (variable — identify the device)

📌

For /25: 25 network bits → subnet mask 255.255.255.128. Block size = 256−128 = 128. Two subnets: .0–.127 and .128–.255. Address 130 falls in the .128 block → Network: 192.168.1.128, Broadcast: 192.168.1.255, First host: 192.168.1.129.

Subnet Calculation — Step by Step

Given: 192.168.10.200/26 — find network, broadcast, usable range, and host count.

1

Identify host bits and usable hosts

/26 → 32 − 26 = 6 host bits. Usable hosts = 2⁶ − 2 = 62 hosts. (Subtract 2 for network and broadcast addresses.)

2

Convert prefix to subnet mask

/26 means first 26 bits are 1, last 6 are 0 → 11111111.11111111.11111111.11000000 → 255.255.255.192

3

Find the block size (magic number)

Block size = 256 − 192 = 64. Subnets increment by 64: .0, .64, .128, .192. Four subnets total from a /24.

4

Find the network address

200 ÷ 64 = 3 remainder 8. The block containing 200 starts at 3 × 64 = 192. Network address: 192.168.10.192

5

Find broadcast and usable range

Broadcast = next block start − 1 = 256 − 1 = 192.168.10.255. Usable range: 192.168.10.193 – 192.168.10.254 (62 hosts).

IPv6 Compression Rules

Two rules reduce verbose 128-bit addresses to readable shorthand.

1

Rule 1 — Drop leading zeros in each group

2001:0db8:0000:0001 → 2001:db8:0:1. Remove leading zeros from every 16-bit group independently. Trailing zeros must remain.

2

Rule 2 — Replace one consecutive run of zero groups with ::

2001:db8:0:0:0:0:0:1 → 2001:db8::1. The :: can only be used once. When two equal-length runs exist, replace the leftmost one per RFC 5952.

⚠️

Common exam trap: Using :: more than once in a single address is invalid. 2001::db8::1 is illegal — the router cannot determine how many zero groups each :: represents.

DHCP DORA Process

D

Discover — Client broadcasts for any DHCP server

Source: 0.0.0.0 (no IP yet). Destination: 255.255.255.255 (broadcast). UDP port 67 (server) and 68 (client).

O

Offer — Server offers an IP address

DHCP server responds with an available IP, subnet mask, default gateway, DNS servers, and lease time. May be unicast or broadcast.

R

Request — Client formally requests the offered IP

Client broadcasts to let all DHCP servers know which offer it accepted (there may be multiple servers). Confirms the IP it wants.

A

Acknowledge — Server confirms the lease

DHCP server sends final confirmation with full configuration. Client begins using the IP. Lease timer starts.

Compare & Reference

Filter by topic to study specific address categories, subnet sizes, IPv4 vs IPv6 differences, and NAT types.

Concept	Range / Value	Details	Key Notes
Class A IPv4	1.0.0.0 – 126.255.255.255	Default mask /8 (255.0.0.0) · ~16.7M hosts per network · Private: 10.0.0.0/8	127.x.x.x is reserved for loopback — NOT a usable Class A range
Class B IPv4	128.0.0.0 – 191.255.255.255	Default mask /16 (255.255.0.0) · 65,534 hosts · Private: 172.16.0.0/12 (172.16–172.31)	172.16–172.31 are private; 172.32+ is public
Class C IPv4	192.0.0.0 – 223.255.255.255	Default mask /24 (255.255.255.0) · 254 hosts · Private: 192.168.0.0/16	Most common in home/small office networks
Class D IPv4	224.0.0.0 – 239.255.255.255	Multicast — one source, multiple receivers · No subnet mask	Not assigned to hosts; used by routing protocols (e.g., OSPF uses 224.0.0.5)
Class E IPv4	240.0.0.0 – 255.255.255.255	Experimental / Reserved · 255.255.255.255 = limited broadcast	Never used in production networks
APIPA IPv4	169.254.0.0 – 169.254.255.255	/16 mask · Auto-assigned by OS when DHCP fails (Zeroconf)	Link-local only — no routing between subnets. Seeing this = DHCP problem.
/24 Subnet	255.255.255.0 · Block: 256	256 addresses · 254 usable hosts · 1 subnet from /24	Standard LAN segment — most common in practice
/25 Subnet	255.255.255.128 · Block: 128	128 addresses · 126 usable hosts · 2 subnets from /24	Subnets: .0–.127 and .128–.255
/26 Subnet	255.255.255.192 · Block: 64	64 addresses · 62 usable hosts · 4 subnets from /24	Subnets: .0, .64, .128, .192
/27 Subnet	255.255.255.224 · Block: 32	32 addresses · 30 usable hosts · 8 subnets from /24	Subnets: .0, .32, .64, .96, .128, .160, .192, .224
/28 Subnet	255.255.255.240 · Block: 16	16 addresses · 14 usable hosts · 16 subnets from /24	Small segments: server rooms, management VLANs
/29 Subnet	255.255.255.248 · Block: 8	8 addresses · 6 usable hosts · 32 subnets from /24	Very small segments; IoT devices or printers
/30 Subnet	255.255.255.252 · Block: 4	4 addresses · 2 usable hosts · 64 subnets from /24	Point-to-point WAN links — exactly 2 endpoints needed
Address Length IPv4 vs IPv6	IPv4: 32 bits · IPv6: 128 bits	IPv4: ~4.3B addresses · IPv6: 340 undecillion	IPv6 was created to solve IPv4 exhaustion
Notation IPv4 vs IPv6	IPv4: dotted decimal · IPv6: colon hex	IPv4: 192.168.1.1 · IPv6: 2001:db8::1	IPv6 can compress consecutive zero groups with ::
Broadcast IPv4 vs IPv6	IPv4: Yes · IPv6: No	IPv6 uses multicast (ff00::/8) instead of broadcast	Eliminates broadcast storms — significant performance benefit
Auto-Config IPv4 vs IPv6	IPv4: APIPA (169.254/16) · IPv6: SLAAC	SLAAC (Stateless Address Autoconfiguration) uses Router Advertisements + EUI-64	IPv6 devices can self-configure without a DHCP server
Loopback IPv4 vs IPv6	IPv4: 127.0.0.1 · IPv6: ::1	Both test the local protocol stack without sending traffic	::1/128 is the full IPv6 loopback notation
Link-Local IPv4 vs IPv6	IPv4: 169.254.0.0/16 · IPv6: fe80::/10	IPv6 fe80:: is auto-configured on every interface, always present	IPv4 APIPA = failure state; IPv6 fe80:: = normal operation
Static NAT NAT	1 private IP ↔ 1 public IP	Permanent, fixed mapping. Used for servers needing consistent inbound access.	Requires one public IP per device — does not conserve addresses
Dynamic NAT NAT	Pool of public IPs shared	Private IPs mapped to available public IPs from a pool. Mapping changes each session.	Limited by pool size — if pool exhausted, new connections fail
PAT / Overload NAT	Many private IPs → 1 public IP	Uses unique port numbers to track sessions. Most common NAT type. Used by virtually all home routers.	Up to ~65,000 simultaneous sessions per public IP

Real Examples

Worked exam-style scenarios — the kind of problems that appear on the N10-009.

✂️ Subnetting a /24 into 4 Equal Subnets

A company is given 192.168.50.0/24. The network administrator must divide it into exactly 4 equal subnets. What is the new prefix length, subnet mask, number of usable hosts per subnet, and the network address of each subnet?

To create 4 subnets from /24: borrow 2 bits (2² = 4 subnets). New prefix = /24 + 2 = /26.

New Prefix/26

Subnet Mask255.255.255.192

Block Size64 (256−192)

Usable Hosts62 per subnet (2⁶−2)

Subnet 1192.168.50.0 – .63

Subnet 2192.168.50.64 – .127

Subnet 3192.168.50.128 – .191

Subnet 4192.168.50.192 – .255

💡

Each subnet's broadcast address is the last address in the block (.63, .127, .191, .255). The first usable host is network+1, the last usable is broadcast−1.

🌍 Identify Address Type and Class

A technician inspects a device showing IP 172.20.100.50 with subnet mask 255.255.0.0. Is this address public or private? What class is it? What is the network address?

Address TypePrivate (RFC 1918)

ClassClass B (128–191 range)

Private Range172.16.0.0/12 (includes 172.16–172.31)

Mask255.255.0.0 = /16

Network Addr172.20.0.0

Broadcast172.20.255.255

⚠️

Exam trap: 172.20.x.x is private, but 172.32.x.x is public. The private B range is strictly 172.16.0.0 through 172.31.255.255 (/12 mask). Many candidates assume all 172.x.x.x addresses are private — they are not.

⚙️ APIPA Troubleshooting Scenario

A user reports their laptop cannot access any network resources. The technician runs ipconfig and sees: IP Address 169.254.45.12, Subnet Mask 255.255.0.0, Default Gateway (blank). What caused this and what should be checked?

Root CauseDHCP failure — OS self-assigned APIPA

APIPA Range169.254.0.0/16

Check 1Is the DHCP server running?

Check 2Is the client's NIC connected to the network?

Check 3Is UDP port 67/68 blocked by a firewall?

Check 4Is the DHCP scope exhausted (no IPs available)?

✅

Quick test: run ipconfig /release then ipconfig /renew. If APIPA persists, the problem is network connectivity or the DHCP server — not the client's configuration.

🔢 IPv6 Address Compression

Compress the full IPv6 address: 2001:0DB8:0000:0000:00AB:0000:0000:0001. Then identify what type of address this is.

Step 1: Drop leading zeros2001:db8:0:0:ab:0:0:1

Step 2: Find longest zero runGroups 3–4 (len 2) tied with 6–7 (len 2) — use leftmost

Compressed Result2001:db8::ab:0:0:1

Address TypeGlobal Unicast (2000::/3) — internet-routable

Common useAssigned by ISP to end devices

Why not fe80::fe80:: = link-local (non-routable)

📌

The :: can only appear once. Since both zero runs (positions 3–4 and 6–7) have equal length, RFC 5952 says replace the leftmost — giving 2001:db8::ab:0:0:1. The trailing 0:0:1 cannot be further compressed.

Practice Quiz

10 scenario-based questions aligned to N10-009 exam style. Each tests a specific IP addressing concept.

Question 1 of 10

IPv4

—

Subnetting

—

IPv6

—

Services

—

Subnet Advisor

Answer a few questions and get instant guidance on subnet calculations, address identification, mask selection, and troubleshooting.

🔧 What do you need help with?

🔢 What is your subnet prefix length?

🔍 Which best describes the address?

📏 How many hosts do you need per subnet?

🛠️ What symptom are you seeing?

Memory Hooks

Click each card to flip it and reveal the answer. Then use the cheat sheet for instant reference.

📶

What IP range does APIPA use?

Tap to reveal

169.254.0.0/16

Auto-assigned when DHCP fails. Link-local only — never routed.

✂️

What formula gives block size?

Tap to reveal

256 − mask octet

/26 → 255.255.255.192 → 256−192 = 64. Subnets increment by 64.

🔒

Name the 3 RFC 1918 private ranges

Tap to reveal

10/8 · 172.16/12 · 192.168/16

Class A · Class B (172.16–172.31) · Class C

📬

What are the 4 DHCP steps?

Tap to reveal

D O R A

Discover → Offer → Request → Acknowledge

🔗

When do you use /30?

Tap to reveal

Point-to-point WAN links

Only 2 usable hosts. Mask: 255.255.255.252. Block size: 4.

🔄

What is the IPv6 loopback?

Tap to reveal

::1/128

Equivalent to IPv4's 127.0.0.1. Tests local IPv6 stack.

🧮

Usable hosts formula?

Tap to reveal

2ʰ − 2

h = host bits (32 − prefix). Subtract 2 for network & broadcast. /26 → 2⁶−2 = 62.

🌐

What IPv6 prefix is link-local?

Tap to reveal

fe80::/10

Auto-configured on every IPv6 interface. Non-routable. Always present — not a failure state (unlike APIPA).

Quick Reference Cheat Sheet

Prefix	Subnet Mask	Usable Hosts	Block Size	Typical Use
/8	255.0.0.0	16,777,214	—	Class A default; 10.0.0.0/8 private
/12	255.240.0.0	1,048,574	—	172.16.0.0/12 private Class B range
/16	255.255.0.0	65,534	—	Class B default; 192.168.0.0/16 private
/24	255.255.255.0	254	256	Standard LAN segment
/25	255.255.255.128	126	128	Split /24 in half
/26	255.255.255.192	62	64	4 subnets from /24
/27	255.255.255.224	30	32	8 subnets from /24
/28	255.255.255.240	14	16	Small VLANs, server rooms
/29	255.255.255.248	6	8	Tiny segments, IoT
/30	255.255.255.252	2	4	Point-to-point WAN links
127.0.0.1	—	—	—	IPv4 loopback (127.0.0.0/8 reserved)
169.254/16	255.255.0.0	—	—	APIPA — DHCP failure indicator
::1/128	—	—	—	IPv6 loopback
fe80::/10	—	—	—	IPv6 link-local (always present)
ff00::/8	—	—	—	IPv6 multicast (replaces broadcast)

IP Addressing & Subnetting

32-Bit Addressing

Dividing Networks

128-Bit Addressing

DHCP, NAT & More

Why IP Addressing Matters

IPv4 Binary Structure — 192.168.1.130/25

Identify host bits and usable hosts

Convert prefix to subnet mask

Find the block size (magic number)

Find the network address

Find broadcast and usable range

Rule 1 — Drop leading zeros in each group

Rule 2 — Replace one consecutive run of zero groups with ::

Discover — Client broadcasts for any DHCP server

Offer — Server offers an IP address

Request — Client formally requests the offered IP

Acknowledge — Server confirms the lease

✂️ Subnetting a /24 into 4 Equal Subnets

🌍 Identify Address Type and Class

⚙️ APIPA Troubleshooting Scenario

🔢 IPv6 Address Compression

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