IP Address Formats Explained: IPv4, IPv6, CIDR & Subnets
Dotted decimal, colon-separated hex, /24 notation — IP addresses come in several formats. Here is how to read each one and what CIDR and subnet masks actually mean.
An IP address can appear in several different-looking formats, and the notation trips up a lot of people. Once you know the pattern behind each one, addresses like 192.168.1.0/24 or 2001:db8::1 stop looking cryptic. Here is how to read every common format.
IPv4: dotted decimal
An IPv4 address is 32 bits, written as four octets — numbers from 0 to 255 — separated by dots, such as 203.0.113.42. Each octet represents eight bits, so the four together make up the full 32-bit address. Because each part maxes out at 255, anything higher (like 256.0.0.1) is invalid. That simple range is the entire reason there are only about 4.3 billion IPv4 addresses — the story behind why the world ran out of them.
IPv6: colon-separated hexadecimal
An IPv6 address is 128 bits, written as eight groups of four hexadecimal digits separated by colons, like 2001:0db8:0000:0000:0000:0000:0000:0001. To keep them manageable, IPv6 allows two shortcuts:
- Drop leading zeros in each group:
0db8becomesdb8. - Collapse one run of all-zero groups into
::. So the address above shortens to2001:db8::1.
The :: can only appear once in an address, otherwise it would be ambiguous. Our IPv4 vs IPv6 guide compares the two schemes in everyday terms.
CIDR notation: the slash
You will often see an address followed by a slash and a number, like 192.168.1.0/24. That is CIDR (Classless Inter-Domain Routing) notation, and the number is the prefix length — how many bits at the start of the address identify the network rather than the individual host.
/24means the first 24 bits are the network, leaving 8 bits for hosts — 256 addresses (254 usable)./16means the first 16 bits are the network — about 65,000 addresses./32refers to a single, exact address.
The smaller the number after the slash, the larger the block of addresses it covers.
Subnet masks
A subnet mask is the older way of expressing the same idea as CIDR. The mask 255.255.255.0 is identical to /24: the 255 octets mark network bits and the 0 marks host bits. Both tell a device which addresses are on its local network and which live elsewhere, so it knows whether to talk directly or hand traffic to the router.
Public, private, and special ranges
Certain blocks are reserved and never appear on the public internet: 192.168.0.0/16, 10.0.0.0/8, and 172.16.0.0/12 are private ranges used inside home and office networks. That is why your device's private IP looks different from the public one on our home page. Blocks are also grouped into networks run by organizations, each identified by an autonomous system number.
The bottom line
IPv4 uses dotted decimal, IPv6 uses shortened hexadecimal, and CIDR's slash notation (mirrored by subnet masks) tells you how big a block of addresses is. None of it is as intimidating as it first looks once you see the structure. To put it into practice, look up any address — in either format — with our IP lookup tool.