The ABCs of IP Addresses

IP addresses are unique sets of numbers assigned to devices that connect to a network. These sets of numbers are used to send and receive information on the network and to find other devices. An IP address is very much like a home address that others use to send you mail and for you to send mail from—it's a unique identifier of your location.
While your home or small business router usually handles all IP address assignments via a DHCP (Dynamic Host Control Protocol) server within the router's software, there are organizations that officially manage and allocate blocks of IP addresses for companies and organizations. IANA (Internet Assigned Number Authority) handles IP addresses allocated globally, typically to Regional Internet Registries. In the Unites States, IP address allocation is managed by ARIN (American Registry for Internet Numbers).
Find the Address
IP addresses are represented as a set of dotted decimal numbers. Go ahead and click on the Start button. In the "Search" or "Run" field type in CMD. At the command prompt (which is the C:\> (there may be extra words after the first >. Type after the last >B.) Type in ipconfig /all.
You will see a screen that looks just like the one below (although you may see different numbers):
IP
Look at the area in the yellow box. Look for the line IPv4 address. You will see the numbers 192.168.3.2. This is the IP address that my wireless router is assigning to my laptop which is connected wirelessly to the router.
This address is used by every other device I have connected on my home network to communicate with my laptop.
Is that the address other networks use to find my laptop on the Internet? No. That is an IP address that is given to me by my ISP. You can find that IP address by going into your router's interface and checking the WAN (Wide Area Network) IP address or by accessing the interface of your cable, DSL, or FIOS modem. You typically don't need to know or deal with the WAN IP address unless you are doing tasks like hosting your own web or email server, and we will save those details for a future article.
In contrast, my 192.168.3.2 address is my LAN (Local Area Network) IP address. It is used for internal networking purposes, meaning it is the way my laptop is identified and forwards and receives traffic between all of the devices connected on my home network.
ABC (and D and E)
IP addresses are broken into different classes.
Class A IP addresses are used for huge networks, like those deployed by Internet Service Providers (ISPs). Class A IP addresses support up to 16 million hosts (hosts are devices that connect to a network (computers, servers, switches, routers, printers…etc.) and a Class A network can be divided into 128 different networks.

Class B IP addresses are used for medium and large-sized networks in enterprises and organizations. They support up to 65,000 hosts on 16,000 individual networks.

Class C addresses are most common and used in small business and home networks. These support up to 256 hosts on each of 2 million networks.

Class D and E addresses are least used. Class D is reserved for a not widely used, and reserved for special cases largely for services and applications to stream audio and video to many subscribers at once. Class E addresses are reserved for research purposes by those responsible for Internet networking and IP address research, management, and development.
Look back at the image above. You will see a line that shows my subnet mask is 255.255.255.0. What does this mean?
Subnets divide networks into groups. You may want to create different groups within a network for giving different users access to different resources, for performance optimization, or for security reasons. Subnet masks tell other devices if a specific device is on a local or remote network, to efficiently route packets.
Most small and home business networks use the subnet mask of 255.255.255.0. Your router will automatically configure the appropriate LAN IP and subnet addressing.
Looking back at the image, you will see my gateway address is 192.168.3.1. In most small business and home networks, the gateway address is typically the IP address assigned to the router. The gateway acts almost like a traffic cop, managing the flow of traffic between the Internet or WAN, and your home or small business network—the LAN.
Beyond the Basics
As network grow in size and capacity, subnetting, gateways, and assigning IP addresses gets more complicated, Then of course, there is the world of IPv6—addresses that are used as the shortage of available IPv4 addresses looms.
However, as a good administrator over your home or small business network, it pays to get familiar with the basics.