IP Address

In simple terms

An IP address is the number a computer is reachable at on a network. The internet uses two flavours: IPv4 (looks like 192.168.1.1) and IPv6 (looks like 2001:0db8::1).

The Visual Map

flowchart LR
  subgraph home["home network 192.168.1.0/24"]
    L["laptop<br/>192.168.1.10"]
    P["phone<br/>192.168.1.11"]
  end
  L --> R
  P --> R
  R["router + NAT<br/>public: 203.0.113.7"] --> I((internet))
  I --> S["server<br/>93.184.216.34"]
  D["DNS: example.com → 93.184.216.34"] -.-> I

More detail

• IPv4 is 32 bits, written as four decimal numbers separated by dots. Total address space: about 4.3 billion — long since exhausted.
• IPv6 is 128 bits, written in hexadecimal blocks. The address space is essentially unlimited for practical purposes.

A modern computer often has both, plus a private address inside your network (handed out by your router) and a public address that the rest of the world sees (your ISP’s gateway). Translating between them is NAT (network address translation).

IP addresses route by prefix: routers look at the leading bits and forward packets toward the destination network, hop by hop. A network is written in CIDR notation — 192.168.1.0/24 means “the 256 addresses sharing the first 24 bits”.

Everything on the internet is named by IP underneath. Names like wikipedia.org only exist because we don’t want to memorise numbers.

Under the Hood

An IPv4 address is just a 32-bit integer; prefixes are bit masks over it:

import ipaddress

net = ipaddress.ip_network("192.168.1.0/24")
addr = ipaddress.ip_address("192.168.1.10")

print(int(addr))                 # 3232235786 — the address as one integer
print(addr in net)               # True — first 24 bits match
print(net.netmask)               # 255.255.255.0
print(net.num_addresses)         # 256

# the same machinery at internet scale
print(ipaddress.ip_address("8.8.8.8") in ipaddress.ip_network("8.8.8.0/24"))  # True

“Longest prefix match” — the heart of every routing decision — is comparing leading bits of exactly these integers.

Engineering Trade-offs

• IPv4 scarcity vs IPv6 migration cost. IPv4 ran out, so addresses are bought, leased, and stretched with NAT. IPv6 fixes scarcity permanently, but both stacks must run side by side for decades — every middlebox, firewall rule, and log pipeline doubled.
• NAT: address sharing vs end-to-end reachability. NAT lets a whole household live behind one public IPv4 address, but breaks the original “any host can reach any host” model — peer-to-peer apps need hole-punching and relay servers to cope.
• Static vs dynamic assignment. Static addresses are predictable (servers, DNS targets) but must be managed; DHCP-assigned dynamic addresses are zero-touch but can change, which is why home-hosted services need dynamic-DNS workarounds.
• Aggregation vs precision in routing. Big prefixes keep the global routing table small; carving them into small announcements gives fine-grained traffic control but bloats every router’s memory on the planet.

Real-world examples

• 127.0.0.1 (and ::1) — your own machine.
• 8.8.8.8 — one of Google’s public DNS servers.
• 192.168.x.x, 10.x.x.x — typical home-router private ranges.
• A single phone on cellular often has three IPs at once: a private IPv4 from the carrier’s NAT, an IPv6 from the cellular network, and a private IPv4 on its local Wi-Fi.

Common misconceptions

• “My IP address never changes.” Many ISPs assign dynamic addresses that change on reconnect. Even “static” cloud IPs can be reassigned if you release them.
• “An IP address identifies a person.” It identifies a network endpoint, often shared via NAT. Tying it to a person needs more evidence.

Try it yourself

See every address your machine holds right now, then do some prefix math:

ip -brief addr show   # Linux/WSL: interface, state, IPv4 + IPv6 addresses

python3 -c "
import ipaddress
net = ipaddress.ip_network('10.0.0.0/8')
print(f'{net} holds {net.num_addresses:,} addresses')
for sub in list(net.subnets(new_prefix=10))[:4]:
    print('  subnet:', sub)
"

Spot the 192.168.x.x or 10.x.x.x private address — that’s the one NAT hides from the internet.

Learn next

• DNS — names are easier than numbers.
• NAT — how many devices share one public address.
• Router — the machines that act on these prefixes.