IPv4 is a type of Internet networking protocol that functions as an address for devices connected to a network. Once connected to a network, an IPv4 address is assigned by the host (usually an Internet service provider, or ISP) to the client (the user's device, be it computer, cell phone or gaming system) as a way of identifying where queried traffic should be routed to.
Format of IPv4
IPv4 addresses are commonly found in the format of xxx.xxx.xxx.xxx or some variation thereof, with each x replaced by a number from one to nine. For example, 192.168.1.1 is a common address used for household routers. These addresses are 32-bit addresses, and the number of unique addresses available using IPv4 is 2^32; this works out mathematically to just under 4.3 billion different combinations possible.
The most common usage of IPv4 addresses is connecting to the Internet. Plugging a computer into a high speed modem will have an IPv4 address assigned directly to that computer from the user's ISP. Using a household router adds an extra step to the process; the ISP in this case assigns the IPv4 address to the router, and the router then acts as a host for computers on the local network by assigning sub-addresses to each. This allows the computers to share the Internet connection.
Diminished Address Availability
IPv4 addresses are inherently limited. While 4.3 billion seems like a large number, many addresses are reserved for special uses. Every device that connects to the Internet is assigned a unique address; this has reduced the number of addresses available as more and more devices connect to the network.
An alternative being developed to solve the problem of diminishing IPv4 addresses is called IPv6. Instead of being in the format of xxx.xxx.xxx.xxx, it offers much longer addressing, which works out to 3.4 x 10^38 different possible combinations under the newer standard. It has not seen widespread adaptation; IPv4 is still the dominant standard for Internet protocol addressing.