What is an IP address
The IP address (Internet Protocol Address) is a number assigned to any device that is connected to IP network. The IP number is used to define the host or network interface and assist in location addressing.
There are two versions of the IP address. One is IPv4 and is a 32 bit number which was the original numbering scheme. The latest is IPv6 which is a 128bit number. Due to large number of devices now connected to the Internet the IPv4 numbering scheme is nearly exhausted. Most computers, service providers have supported IPv6 for many years now. However the Arduino Ethernet devices do not support IPv6.
What are private IP addresses
There are three ranges of IPv4 addresses that have been reserved for private use and are not routed on the internet. Normally when we setup a network within our house or business we would use one of the three ranges for our ip address. Most broadband routers that use DHCP would automatically default to a private address range. When you connect to the Internet the IP that would be used is the public IP address assigned to your router by your internet service provider. Through a process call Network Address Translation (NAT) it allows you to access the internet using you private IP address.
What is are static IP address
We can configure a network device using either a static or dynamically assigned IP address. Normally we would assign a static (fixed) ip addresses to things such as servers, email servers, routers. These days most devices such as desktops, laptops, phones, ipads, gaming consoles etc would be assigned a dynamic address which could change regularly. For our Arduino and other platforms we can use a static or dynamically assigned IP address. For Arduino ethernet projects that we want to connect to we would use a static (fixed) ip address. For projects that use an Arduino to just send data to a webserver then we could use a dynamically assigned IP address that would normally be assigned using a DHCP server.
When assigning static ip addresses in our private network we need to keep track of what we assign them to. You want to avoid assigning the same IP address to more than one device as it will not work.