Understanding TCP/IP Networking: IP Addresses, Subnets, and Routing

Why Developers Should Understand TCP/IP

Every application you build communicates over a network. Whether you’re configuring cloud infrastructure, debugging connectivity issues, setting up VPCs, or understanding why a request times out, TCP/IP knowledge is foundational. This article breaks down the core concepts: IP addresses, subnet masks, network classes, default gateways, and private address ranges.

IP Addresses: Network and Host

An IP address is a 32-bit number that uniquely identifies a device on a TCP/IP network. It’s written in dotted-decimal notation — four numbers (octets) separated by periods:

192.168.123.132

Each octet represents 8 bits, so the full address in binary is:

11000000.10101000.01111011.10000100

Two Parts of Every IP Address

Every IP address is split into two logical parts:

Part	Purpose	Example
Network address	Identifies which network the device belongs to	192.168.123.0
Host address	Identifies the specific device within that network	0.0.0.132

Routers use the network portion to forward packets to the correct network. Once the packet arrives at the destination network, the host portion identifies the specific device.

This is analogous to postal mail: the network address is the street name, and the host address is the house number.

Subnet Masks: Separating Network from Host

The division between network and host isn’t fixed — it’s determined by the subnet mask.

A subnet mask is another 32-bit number. In binary, it’s always a sequence of 1s followed by 0s:

• The 1 bits mark the network portion
• The 0 bits mark the host portion

Example

11000000.10101000.01111011.10000100  — IP address   (192.168.123.132)
11111111.11111111.11111111.00000000  — Subnet mask  (255.255.255.0)

The first 24 bits (all 1s in the mask) are the network address. The remaining 8 bits (all 0s) are the host address:

11000000.10101000.01111011.00000000  — Network address (192.168.123.0)
00000000.00000000.00000000.10000100  — Host address    (0.0.0.132)

With this mask, any device with an IP starting with 192.168.123.x is on the same network. The last octet (0–255) identifies individual hosts within it.

How the Mask Works (Bitwise AND)

To extract the network address, perform a bitwise AND between the IP address and the subnet mask:

  192.168.123.132   →  11000000.10101000.01111011.10000100
& 255.255.255.0     →  11111111.11111111.11111111.00000000
= 192.168.123.0     →  11000000.10101000.01111011.00000000

This operation zeroes out the host bits, leaving only the network address.

Common Subnet Masks

Decimal	Binary	Network bits	Hosts per subnet
255.0.0.0	11111111.00000000.00000000.00000000	8	16,777,214
255.255.0.0	11111111.11111111.00000000.00000000	16	65,534
255.255.255.0	11111111.11111111.11111111.00000000	24	254
255.255.255.192	11111111.11111111.11111111.11000000	26	62
255.255.255.224	11111111.11111111.11111111.11100000	27	30

The number of usable hosts is 2^(host bits) - 2 (subtract the network address and broadcast address).

CIDR Notation

Instead of writing out the full subnet mask, CIDR notation appends the number of network bits after a slash:

CIDR	Subnet Mask	Meaning
/8	255.0.0.0	First 8 bits are the network
/16	255.255.0.0	First 16 bits are the network
/24	255.255.255.0	First 24 bits are the network
/26	255.255.255.192	First 26 bits are the network
/32	255.255.255.255	Single host

192.168.123.0/24 means “the network 192.168.123.0 with a 24-bit mask” — equivalent to subnet mask 255.255.255.0.

You’ll see CIDR notation everywhere in cloud configurations (AWS VPCs, security groups, route tables).

Network Classes

Historically, IP addresses were grouped into classes based on their first octet. While classful addressing has been largely replaced by CIDR, the terminology persists and is useful to understand:

Class	First Octet Range	Default Subnet Mask	Network/Host Split	Example
A	1–126	255.0.0.0 (/8)	8 / 24 bits	10.52.36.11
B	128–191	255.255.0.0 (/16)	16 / 16 bits	172.16.52.63
C	192–223	255.255.255.0 (/24)	24 / 8 bits	192.168.123.132

Class A networks have few network addresses but millions of hosts per network. Class C networks have many network addresses but only 254 hosts each. Class B sits in between.

Note: 127.x.x.x is reserved for loopback (127.0.0.1 is localhost). Classes D (224–239, multicast) and E (240–255, experimental) exist but aren’t used for standard addressing.

Default Gateways

When a device wants to communicate with another device, it first determines whether the destination is local (same subnet) or remote (different subnet).

The process:

1. Apply the subnet mask to both the source IP and destination IP (bitwise AND)
2. Compare the resulting network addresses
3. If they match — the destination is local; send directly on the subnet
4. If they don’t match — the destination is remote; forward the packet to the default gateway

The default gateway is a router on your local subnet that knows how to forward packets to other networks. It’s the “exit door” from your subnet to the rest of the network.

Example

Your device: 192.168.1.100 with mask 255.255.255.0 and gateway 192.168.1.1

Sending to 192.168.1.50 (local):

192.168.1.100 AND 255.255.255.0 = 192.168.1.0
192.168.1.50  AND 255.255.255.0 = 192.168.1.0
→ Same network → send directly

Sending to 10.0.0.5 (remote):

192.168.1.100 AND 255.255.255.0 = 192.168.1.0
10.0.0.5      AND 255.255.255.0 = 10.0.0.0
→ Different network → forward to gateway 192.168.1.1

The router at 192.168.1.1 then consults its routing table to determine where to send the packet next.

Private IP Addresses

Not every device needs a globally unique, publicly routable IP address. RFC 1918 reserves three address ranges for private networks:

Name	CIDR Block	Address Range	Addresses	Classful Description
24-bit block	10.0.0.0/8	10.0.0.0 – 10.255.255.255	16,777,216	Single Class A
20-bit block	172.16.0.0/12	172.16.0.0 – 172.31.255.255	1,048,576	16 Class B blocks
16-bit block	192.168.0.0/16	192.168.0.0 – 192.168.255.255	65,536	256 Class C blocks

Key properties of private addresses

• Not routable on the public internet — routers discard packets with private source/destination addresses
• Free to use — no registration required
• Reusable — different organisations can use the same private ranges without conflict
• Require NAT for internet access — Network Address Translation maps private addresses to a public IP for outbound traffic

Where you’ll see them

• Home networks: Your router typically assigns addresses from 192.168.0.0/24 or 192.168.1.0/24
• AWS VPCs: Default VPC uses 172.31.0.0/16; custom VPCs commonly use 10.0.0.0/16
• Docker networks: Default bridge uses 172.17.0.0/16
• Kubernetes: Pod networks typically use 10.244.0.0/16 or similar

IPv6: The Future (and Present)

IPv4’s 32-bit address space provides approximately 4.3 billion addresses — not enough for the modern internet. IPv6 expands the address size to 128 bits, providing roughly 3.4 x 10^38 addresses.

IPv6 addresses are written as eight groups of four hexadecimal digits:

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

Leading zeros can be omitted, and consecutive groups of zeros can be replaced with :::

2001:db8:85a3::8a2e:370:7334

While IPv6 adoption continues to grow, most developers still work primarily with IPv4 in cloud and enterprise environments. Understanding IPv4 thoroughly remains essential.

Quick Reference

Concept	Key Point
IP address	32-bit number identifying a device on a network
Subnet mask	Defines which bits are network vs. host
CIDR notation	Shorthand for subnet mask (/24 = 255.255.255.0)
Network classes	Historical grouping by first octet (A: 1–126, B: 128–191, C: 192–223)
Default gateway	Router that forwards packets to other networks
Private addresses	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — not internet-routable
NAT	Translates private addresses to public for internet access
IPv6	128-bit addresses replacing IPv4’s 32-bit space

References

• RFC 791 — Internet Protocol
• RFC 1918 — Private Address Space
• CIDR — Classless Inter-Domain Routing
• IPv6 — Wikipedia