Concept · 7 min read
What is an IP address?
Why does every machine need one, and what does the number actually mean?
An IP address identifies an interface on a network so packets can be delivered to the right place. It is not a person, not a building and not a permanent label for a machine.
The one-sentence version
An IP address is a numeric label assigned to a network interface. Your machine sends packets to a destination address; every router between you and it only needs to answer 'which way is closer to this number?' That single idea explains addressing, routing and most of subnetting.
IPv4 in practice
An IPv4 address is 32 bits written as four decimal octets. The number alone is not enough — you also need the prefix length, which says how many leading bits identify the network.
text
192.0.2.10/24 → network 192.0.2.0 – 192.0.2.255 10.20.0.5/16 → network 10.20.0.0 – 10.20.255.255
Private ranges you will meet constantly
Three IPv4 ranges are reserved for internal use and must not be routed on the public internet. Anything else is potentially public.
- 10.0.0.0/8 — one huge private block, common in data centres
- 172.16.0.0/12 — frequently used by container and cloud networks
- 192.168.0.0/16 — home and small-office routers
- 169.254.0.0/16 — link-local: means 'no address was assigned', which is a diagnostic in itself
IPv6, briefly
128 bits, written in hexadecimal groups, with no NAT expectation — every interface gets a globally reachable address unless a firewall says otherwise. Prefixes are usually /64 per subnet, and the low 64 bits often encode the MAC address (a privacy concern worth knowing about).
What an address does not tell you
- It is not identity: shared NAT, DHCP and dynamic assignments mean the same address belongs to different endpoints over time.
- Geolocation from an address is an estimate from registration data plus measurements, not a location reading.
- Reverse DNS can be absent, wrong or deliberately set — corroboration required.
Check your understanding
Answer before expanding. If you cannot explain it in one sentence, the section above needs a re-read.
Two hosts on 192.168.1.10/24 and 192.168.1.20/16 — same subnet?Q1
Yes, on the /16 host's view they are in the same network; the /24 host will still send off-subnet traffic to its default gateway. Mismatched prefix lengths are a classic local cause of asymmetric routing.
You see 169.254.x.x in a capture. What does it suggest?Q2
The interface failed to obtain an address (DHCP or SLAAC failure) and self-assigned a link-local one. Traffic beyond the local segment should not exist.
Where this shows up
Tools in the directory whose commands assume this knowledge.