IPv4
IPv4 is the fourth version of the Internet Protocol, using 32-bit addresses written as four numbers from 0 to 255, such as 203.0.113.7.
What IPv4 is
IPv4 is the addressing scheme that most of the internet has used since the early 1980s. An IPv4 address is a 32-bit number, written for humans as four decimal groups separated by dots. Each group holds one byte, so it ranges from 0 to 255. 203.0.113.7 is a valid address; 256.1.1.1 is not.
Thirty-two bits allow about 4.3 billion addresses. That sounded enormous in 1981 and ran out in the 2010s: the regional registries have no large free blocks left, and addresses are now bought, leased, and shared. The long-term answer is IPv6, but IPv4 remains what most websites, proxies, and VPN services actually use.
Ranges and reserved blocks
Addresses are grouped into networks using CIDR notation or a subnet mask. Some blocks never appear on the public internet:
10.0.0.0/8,172.16.0.0/12, and192.168.0.0/16are private ranges used behind NAT.127.0.0.0/8is loopback (your own machine).100.64.0.0/10is shared address space used by carriers for large-scale NAT.169.254.0.0/16is link-local, assigned when no DHCP server answers.
If a tool shows one of these as your address, it is reporting a local interface, not what websites see.
Why it matters for proxy and VPN users
Because IPv4 space is scarce, a single address is often shared by many users, and the reputation of that address follows everyone on it. Hosting ranges, mobile carrier pools, and residential blocks are all identifiable by their owning network, which is why IP lookup shows the ASN next to the address. A clean IPv4 address on a consumer ISP range is what most websites consider normal.
How to check
IP lookup shows your address with its decimal, hexadecimal, binary, and reverse-DNS forms. The subnet calculator expands any CIDR range into its first and last address, mask, and host count. The IPv6 test tells you whether your connection also has IPv6, which matters because a tunnel that carries only IPv4 can leak.