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Bhanu Bisht

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IP Addressing & Subnetting

IPv4, Subnetting Math, CIDR Notation, IPv6 & NAT

IP Addressing: The logical layer-3 addressing system that allows packets to route across global networks.

IPv4 Addressing Architecture

32-bit Logical Addressing System

Structure of IPv4

An IPv4 address is a 32-bit numerical label divided into 4 octets (8 bits each), separated by dots. It provides a total address space of $2^{32} \approx 4.29 \text{ billion}$ addresses.

IPv4 Classful Addressing Table:

Class	First Octet	Address Range	Default Subnet Mask	Usable Hosts / Subnet
Class A	1 – 126	1.0.0.0 – 126.255.255.255	255.0.0.0 (`/8`)	16,777,214
Class B	128 – 191	128.0.0.0 – 191.255.255.255	255.255.0.0 (`/16`)	65,534
Class C	192 – 223	192.0.0.0 – 223.255.255.255	255.255.255.0 (`/24`)	254
Class D	224 – 239	224.0.0.0 – 239.255.255.255	N/A (Multicast)	Reserved
Class E	240 – 255	240.0.0.0 – 255.255.255.255	N/A (Experimental)	Reserved

IPv4 Address Interface Output

# Linux System IP Configuration & Routing Information
$ ip -4 addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP
    inet 192.168.1.105/24 brd 192.168.1.255 scope global dynamic eth0
       valid_lft 86142sec preferred_lft 86142sec

$ ip route show
default via 192.168.1.1 dev eth0 proto dhcp src 192.168.1.105 metric 100
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.105 metric 100

Private IP Address Ranges (RFC 1918)

Private vs Public Address Space

To prevent global IPv4 address exhaustion, RFC 1918 reserved dedicated address blocks for internal private networks. Private IP addresses are non-routable on the public Internet and rely on NAT for outbound internet access.

RFC 1918 Private Ranges:

Class	Private Address Range	CIDR Prefix	Total Addresses
Class A	10.0.0.0 – 10.255.255.255	10.0.0.0/8	16,777,216
Class B	172.16.0.0 – 172.31.255.255	172.16.0.0/12	1,048,576
Class C	192.168.0.0 – 192.168.255.255	192.168.0.0/16	65,536

Private IP Routing Check

# RFC 1918 Private IP Routing Test & Socket Interface
$ ping -c 3 10.0.0.1
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=1.42 ms
64 bytes from 10.0.0.1: icmp_seq=2 ttl=64 time=0.98 ms

--- 10.0.0.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms

Subnetting & CIDR Notation

Classless Inter-Domain Routing (CIDR)

Subnetting divides a single broad network into smaller, isolated subnetworks. CIDR replaces fixed classful boundaries with variable-length prefix masks (/24, /26), optimizing routing table efficiency and security containment.

CIDR Subnet Reference Table:

Prefix	Subnet Mask	Usable Hosts / Subnet	Wildcard Mask
`/24`	255.255.255.0	254 Hosts	0.0.0.255
`/25`	255.255.255.128	126 Hosts	0.0.0.127
`/26`	255.255.255.192	62 Hosts	0.0.0.63
`/27`	255.255.255.224	30 Hosts	0.0.0.31
`/28`	255.255.255.240	14 Hosts	0.0.0.15
`/30`	255.255.255.252	2 Hosts (Point-to-Point)	0.0.0.3

C Subnet Mask & Host Calculation Code

// C Subnet Mask & Host Range Calculation
#include <stdio.h>
#include <arpa/inet.h>

void calculate_subnet(const char* ip_str, int prefix) {
    uint32_t ip, mask, net, bcast;
    inet_pton(AF_INET, ip_str, &ip);
    ip = ntohl(ip);
    
    mask = prefix == 0 ? 0 : (~0U << (32 - prefix));
    net = ip & mask;
    bcast = net | ~mask;
    
    printf("Network ID : %u.%u.%u.%u\n", (net>>24)&0xFF, (net>>16)&0xFF, (net>>8)&0xFF, net&0xFF);
    printf("Broadcast  : %u.%u.%u.%u\n", (bcast>>24)&0xFF, (bcast>>16)&0xFF, (bcast>>8)&0xFF, bcast&0xFF);
}

Practical Subnet Breakdown Output

# Subnet Partitioning Command (ipcalc)
$ ipcalc 192.168.1.0/26
Address:   192.168.1.0          11000000.10101000.00000001.00 000000
Netmask:   255.255.255.192 = 26 11111111.11111111.11111111.11 000000
Wildcard:  0.0.0.63             00000000.00000000.00000000.00 111111
=>
Network:   192.168.1.0/26       HostMin: 192.168.1.1   HostMax: 192.168.1.62
Broadcast: 192.168.1.63         Hosts/Net: 62

IPv6 Architecture

128-Bit Next-Generation Addressing

IPv6 expands the address space to $2^{128} \approx 3.4 \times 10^{38}$ addresses, featuring built-in IPsec security and auto-configuration (SLAAC).

IPv6 Scope Address Table:

Scope Type	Address Prefix	Functionality
Global Unicast	`2000::/3`	Globally routable public IP address
Link-Local	`fe80::/10`	Auto-configured local LAN communication
Unique Local	`fc00::/7`	Private internal network routing
Multicast	`ff00::/8`	One-to-many packet distribution
Loopback	`::1/128`	Localhost interface

IPv6 Interface Configuration Output

# Linux IPv6 Interface Address Query & Ping6
$ ip -6 addr show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP
    inet6 2001:db8:85a3::8a2e:370:7334/64 scope global dynamic 
       valid_lft 86400sec preferred_lft 14400sec
    inet6 fe80::a00:27ff:fe4e:66a1/64 scope link 
       valid_lft forever preferred_lft forever

Network Address Translation (NAT / PAT)

Port Address Translation (PAT)

NAT translates private IP addresses to a public IP address as packets traverse the router boundary to the Internet.

Linux iptables NAT Setup & Rules

# Linux iptables Port Address Translation (PAT / Masquerade) Setup
$ sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
$ sudo iptables -t nat -L -n -v
Chain POSTROUTING (policy ACCEPT)
 target     prot opt in     out     source               destination         
 MASQUERADE all  --  *      eth0    192.168.1.0/24       0.0.0.0/0

Class

First Octet

Address Range

Default Subnet Mask

Usable Hosts / Subnet

Class A

1 – 126

1.0.0.0 – 126.255.255.255

255.0.0.0 (/8)

16,777,214

Class B

128 – 191

128.0.0.0 – 191.255.255.255

255.255.0.0 (/16)

65,534

Class C

192 – 223

192.0.0.0 – 223.255.255.255

255.255.255.0 (/24)

254

Class D

224 – 239

224.0.0.0 – 239.255.255.255

N/A (Multicast)

Reserved

Class E

240 – 255

240.0.0.0 – 255.255.255.255

N/A (Experimental)

Reserved

# Linux System IP Configuration & Routing Information $ ip -4 addr show eth0 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP inet 192.168.1.105/24 brd 192.168.1.255 scope global dynamic eth0 valid_lft 86142sec preferred_lft 86142sec $ ip route show default via 192.168.1.1 dev eth0 proto dhcp src 192.168.1.105 metric 100 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.105 metric 100

Private IP Address Ranges (RFC 1918)

Private vs Public Address Space

RFC 1918 Private Ranges:

Class	Private Address Range	CIDR Prefix	Total Addresses
Class A	10.0.0.0 – 10.255.255.255	10.0.0.0/8	16,777,216
Class B	172.16.0.0 – 172.31.255.255	172.16.0.0/12	1,048,576
Class C	192.168.0.0 – 192.168.255.255	192.168.0.0/16	65,536

Private IP Routing Check

# RFC 1918 Private IP Routing Test & Socket Interface
$ ping -c 3 10.0.0.1
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=64 time=1.42 ms
64 bytes from 10.0.0.1: icmp_seq=2 ttl=64 time=0.98 ms

--- 10.0.0.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms

Prefix

Subnet Mask

Usable Hosts / Subnet

Wildcard Mask

/24

255.255.255.0

254 Hosts

0.0.0.255

/25

255.255.255.128

126 Hosts

0.0.0.127

/26

255.255.255.192

62 Hosts

0.0.0.63

/27

255.255.255.224

30 Hosts

0.0.0.31

/28

255.255.255.240

14 Hosts

0.0.0.15

/30

255.255.255.252

2 Hosts (Point-to-Point)

0.0.0.3

// C Subnet Mask & Host Range Calculation #include <stdio.h> #include <arpa/inet.h> void calculate_subnet(const char* ip_str, int prefix) { uint32_t ip, mask, net, bcast; inet_pton(AF_INET, ip_str, &ip); ip = ntohl(ip); mask = prefix == 0 ? 0 : (~0U << (32 - prefix)); net = ip & mask; bcast = net | ~mask; printf("Network ID : %u.%u.%u.%u\n", (net>>24)&0xFF, (net>>16)&0xFF, (net>>8)&0xFF, net&0xFF); printf("Broadcast : %u.%u.%u.%u\n", (bcast>>24)&0xFF, (bcast>>16)&0xFF, (bcast>>8)&0xFF, bcast&0xFF); }

# Subnet Partitioning Command (ipcalc) $ ipcalc 192.168.1.0/26 Address: 192.168.1.0 11000000.10101000.00000001.00 000000 Netmask: 255.255.255.192 = 26 11111111.11111111.11111111.11 000000 Wildcard: 0.0.0.63 00000000.00000000.00000000.00 111111 => Network: 192.168.1.0/26 HostMin: 192.168.1.1 HostMax: 192.168.1.62 Broadcast: 192.168.1.63 Hosts/Net: 62