IP addresses & subnets
Key takeaways
An IP address is a number that identifies a device on a network so packets
can be delivered to it. There are two versions in use: IPv4, written as four
numbers like 192.0.2.10, and the much larger IPv6. Addresses split into
public ones, reachable across the internet, and private ones reused
inside home and office networks. A subnet is a block of addresses, and
CIDR notation like /24 says how much of the address is the network part.
New to networks? Start with what is a network?.
Once you know a network is just nodes and links sharing data, the next question is how any one device gets found among millions of others. The answer is addressing, and it’s the foundation the rest of this module builds on.
What an IP address is
An IP address is a number that identifies a device on a network so that data can be routed to the right place. It plays the same role a postal address plays for a letter: when the network breaks your data into packets, each packet carries a destination address, and the routers along the way use it to move the packet closer to that device.
Every device that takes part in a network — your laptop, your phone, a server, the router in your hallway — has at least one IP address. Without it, there would be no way to say where a packet should go, and delivery would be impossible.
IPv4 and IPv6
There are two versions of IP addressing in everyday use.
IPv4 is the original and still the most common. An IPv4 address is 32 bits,
written as four numbers separated by dots — a dotted quad — where each number
ranges from 0 to 255, like 192.0.2.10. That format allows about four billion
distinct addresses. When IPv4 was designed that seemed like plenty, but with
billions of phones, computers, and connected gadgets, the world ran out.
IPv6 exists to solve that shortage. An IPv6 address is 128 bits, written as groups of hexadecimal digits separated by colons, and the space it provides is staggeringly larger — enough that running out is no longer a practical concern. The addresses are longer and less friendly to read, which is one reason we lean on names (covered in the DNS lesson) rather than typing them by hand.
The two coexist. You don’t have to choose one; most modern devices and networks speak both at once, using IPv6 where it’s available and IPv4 where it isn’t. For learning the concepts, IPv4’s dotted quad is the easier picture to hold in your head, so the examples here use it.
Public vs. private addresses
Not every IP address is meant to be reachable from anywhere.
A public address is globally routable — it’s unique across the whole internet, and packets from anywhere can be directed to it. The address of a public website or a mail server is a public address.
A private address comes from one of three reserved ranges set aside for use inside local networks:
10.0.0.0/8172.16.0.0/12192.168.0.0/16
These ranges are reused on countless separate home and office networks at the
same time — the 192.168.1.x addresses on your network and on your neighbour’s
are completely independent. Because they aren’t unique across the internet,
private addresses aren’t reachable directly from the outside. Something has to
translate between the private addresses inside a network and the single public
address the network shows to the world; that trick is
NAT, and it gets its own lesson.
Subnets and CIDR
A subnet is simply a block of addresses that belong to the same local network. Splitting the whole address space into subnets is how large networks stay organised and how routers know which addresses live where.
To describe a subnet compactly we use CIDR notation: an address followed by a
slash and a number, like /24. That number says how many bits at the front of
the address are the network part — shared by every device in the subnet — while
the remaining bits are the host part that identifies individual devices.
Take 192.168.1.0/24. The /24 means the first 24 bits (the 192.168.1
portion) are fixed as the network part, leaving the last 8 bits free for hosts.
So this subnet covers the addresses 192.168.1.0 through 192.168.1.255 — a
single small network of 256 addresses, of which the usable ones sit in between. A
smaller number after the slash, like /16, fixes fewer bits and so covers a
larger block; a larger number covers a smaller one. That’s as far into the math
as you need to go for now: fewer network bits means a bigger block.
How a device gets one
A device can get its IP address in one of two ways.
A static address is set by hand — you configure the device with a specific address and it keeps it. Servers and some fixed equipment are often given static addresses so they’re always found in the same place.
A dynamic address is handed out automatically when the device joins the network, so you don’t have to configure anything. This is what happens the moment your phone connects to Wi-Fi — a service quietly assigns it an address from the local subnet. That service is DHCP, and the DHCP & joining a local network lesson walks through exactly how it works.
Quick check: which of these is a private address?
Recap
- An IP address is a number that identifies a device so packets can be routed to it.
- IPv4 is a dotted quad like
192.0.2.10with about four billion addresses; IPv6 is far larger, and the two coexist. - Public addresses are globally routable; private ranges (
10.0.0.0/8,172.16.0.0/12,192.168.0.0/16) are reused inside local networks and aren’t reachable directly. - A subnet is a block of addresses; CIDR like
/24says how many bits are the network part vs. the host part. 192.168.1.0/24covers192.168.1.0–192.168.1.255— one small network.- A device gets its address either statically (by hand) or dynamically (handed out automatically).
Next up: DNS — names to addresses