IP address

An IP address is the numeric identifier assigned to each device on a TCP/IP network. Two versions coexist: IPv4 and IPv6.

IPv4: 32-bit address, written as four decimal numbers separated by dots, each 0-255. Example: 192.168.1.1. Total address space: ~4.3 billion addresses, exhausted globally around 2011-2015. Modern networks rely heavily on NAT (Network Address Translation) to share a small pool of public IPv4 addresses across many devices.

IPv6: 128-bit address, written as eight groups of four hex digits separated by colons. Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (often abbreviated with :: for consecutive zero groups). Total address space: ~3.4 × 10³⁸ — effectively infinite. Designed in the 1990s to replace IPv4 but adoption has been slow; as of 2026 about 45% of internet traffic uses IPv6.

Reserved address ranges:

• Private (RFC 1918, IPv4): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Used inside home and office networks; never routed on the public internet.
• Loopback: 127.0.0.1 (IPv4), ::1 (IPv6). Refers to “this machine”.
• Link-local (IPv6): fe80::/10. Used for autoconfiguration on a single network segment.

For developers: server logs typically record client IPs. The exact IP your application sees depends on whether traffic passes through proxies/load balancers — check the X-Forwarded-For header if your server isn’t the public-facing edge.

Worked example

A home router with a public IPv4 of 203.0.113.42 serves a private network 192.168.1.0/24. Three devices attach: a laptop at 192.168.1.10, a phone at 192.168.1.11, a smart TV at 192.168.1.12. All three send simultaneous requests to example.com. The router rewrites each outbound packet’s source: laptop’s 192.168.1.10:51234 becomes 203.0.113.42:60001; phone’s 192.168.1.11:48999 becomes 203.0.113.42:60002; TV’s becomes 203.0.113.42:60003. The router keeps a NAT table mapping each external port back to the originating private address. When example.com’s reply arrives at 203.0.113.42:60001, the router looks up 60001 in the table and forwards to 192.168.1.10:51234 — the laptop. This NAT trick is what lets billions of devices share the ~4 billion IPv4 addresses; the cost is that incoming connections from the internet can’t reach a specific device unless explicit port-forwarding is configured.

When and why it matters

IP addresses matter every time a service handles network traffic: rate limiting (by source IP), geolocation (mapping IP to country/city for content licensing or compliance), abuse blocking (DDoS mitigation, banning malicious sources), and access control (allow-listing corporate VPN ranges). The mistakes to avoid: (1) treating the IP in request.remote_addr as the client’s real IP when traffic passes through a load balancer or CDN — you need to consult X-Forwarded-For or Cloudflare’s CF-Connecting-IP header; (2) treating IPv4 addresses as 32-bit integers without considering IPv6 — a service that stores user-IPs in a 32-bit column silently breaks the moment IPv6 traffic arrives; (3) forgetting that mobile carriers use carrier-grade NAT — many subscribers share a single public IP, so per-IP rate limiting can punish entire neighbourhoods. Reference: RFC 791 — Internet Protocol (IPv4).

CIDR notation — the slash format you’ll see everywhere: 10.0.0.0/8 means “the first 8 bits are the network prefix” — addresses from 10.0.0.0 to 10.255.255.255. CIDR (Classless Inter-Domain Routing) replaced the older class-A/B/C scheme in 1993 and is universal in routing tables, firewall rules, and cloud-VPC subnet definitions. For IPv6, the same notation applies but with much larger prefixes: 2001:db8::/32 covers 2⁹⁶ addresses (more than the entire IPv4 internet, in a single block). AWS, GCP, and Azure all carve VPCs out of /16 CIDR blocks by default — large enough that you rarely need to think about subnet exhaustion.

IPv6 transition — why it took thirty years and counting: IPv6 was standardised in 1998. Why is adoption still under 50%? Several reasons compound: home routers had to be upgraded (a slow consumer-hardware refresh cycle); ISPs had to commission new equipment (capital expenditure they preferred to defer); NAT made IPv4 exhaustion bearable in practice, removing the burning-platform incentive; and dual-stack deployment (running both IPv4 and IPv6 simultaneously) is operationally more complex than single-stack. The push is now coming from mobile carriers — T-Mobile, Verizon, Reliance Jio operate IPv6-only mobile networks with 464XLAT translation for legacy IPv4 services. By 2030, the major mobile networks expect to be majority-IPv6, which will finally tip the residual desktop laggards. Reference: RFC 8200 — Internet Protocol, Version 6 (IPv6).