IPv4/IPv6 Addressing, Subnetting & CIDR

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1. IPv4 Addressing

An IPv4 address is a 32-bit number, typically written in dotted-decimal notation (4 octets separated by dots):

192.168.10.50 = 11000000.10101000.00001010.00110010

IPv4 Address Classes (Classful — Legacy)

Special note: 127.x.x.x is reserved for loopback (localhost testing).

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2. Subnet Mask and CIDR Notation

A subnet mask identifies which bits of an IP address belong to the network and which to the host:

• Binary 1 bits = network portion
• Binary 0 bits = host portion

CIDR notation (Classless Inter-Domain Routing) uses /prefix-length to indicate the number of network bits:

• 192.168.1.0/24 = subnet mask 255.255.255.0
• 10.0.0.0/8 = subnet mask 255.0.0.0
• 172.16.0.0/12 = subnet mask 255.240.0.0

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3. Subnetting Worked Example

Problem: A company has the network 192.168.10.0/24 and needs 4 equal subnets. Find:

1. The new subnet mask
2. The subnet addresses
3. The usable host range for each subnet
4. The broadcast address for each subnet

Solution:

Step 1: We need 4 subnets. Since 2^n ≥ 4, we need n = 2 additional subnet bits.

• Original prefix: /24 → New prefix: /24 + 2 = /26
• New subnet mask: /26 = 255.255.255.192

Step 2: Block size = 256 − 192 = 64 (the last octet increments by 64 for each subnet)

Step 3: Calculate the 4 subnets:

Formula: Usable hosts per subnet = 2^(host bits) − 2 = 2^6 − 2 = 62 (Subtract 2 for network address and broadcast address)

Exam Tip: The subnetting formula: Hosts = 2^h − 2 where h = number of host bits. Subnets = 2^s where s = number of borrowed bits. Always subtract 2 from hosts (network ID + broadcast). Very common exam question — practice until it's automatic.

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4. Private IP Address Ranges (RFC 1918)

Private addresses are not routable on the public internet — they require NAT to communicate outside their network.

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5. IPv6 Addressing

IPv6 Address Format

128 bits, written as 8 groups of 4 hexadecimal digits separated by colons:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Simplification rules:

1. Leading zeros: Remove within each group → 2001:db8:85a3:0:0:8a2e:370:7334
2. Consecutive all-zero groups: Replace with :: (only once) → 2001:db8:85a3::8a2e:370:7334

IPv6 Address Types

IPv6 has no broadcast — replaced by multicast (e.g., FF02::1 = all hosts on link).

SLAAC (Stateless Address Autoconfiguration)

IPv6 hosts can configure their own global address without DHCP:

1. Generate a link-local address (FE80:: + EUI-64 from MAC)
2. Receive Router Advertisement with network prefix from router
3. Combine prefix + interface identifier → global unicast address

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6. ARP (Address Resolution Protocol)

ARP maps a known IP address to an unknown MAC address within the same subnet.

ARP Cache: Entries expire after a timeout (typically 2–20 minutes). arp -a displays the cache.

Gratuitous ARP: A host announces its own IP-to-MAC mapping proactively (used for duplicate detection and cache update after IP change).

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Study Deep: Subnetting in Practice

• VLSM (Variable-Length Subnet Masking): Allows different subnets within the same network to have different sizes — assign /30 (2 hosts) to router-to-router links, /24 (254 hosts) to workstation LANs, etc. More efficient than equal-size subnetting.
• Route summarisation: Contiguous subnets can be summarised into a single routing table entry, reducing router memory and CPU usage — critical for scalability.
• IPv6 transition: Dual-stack (both IPv4 and IPv6 running simultaneously) and tunnelling (encapsulating IPv6 in IPv4 packets) are the main transition mechanisms. Most major internet sites now support both.