Because you have a DHCP server in the network.
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix. For example, all devices with IP addresses that start with 100.100.100. would be part of the same subnet. Dividing a network into subnets is useful for both security and performance reasons. IP networks are divided using a subnet mask.
DHCP Dynamic Host Configuration Protocal
Depends on the subnet. For the subnet of 192.168.1.0, the first usable IP address is 192.168.1.1. This is typical of a default wireless router setup. Valid usable IP addresses under this scenario is 192.168.1.1 to 192.168.1.254.
255.255.255.0
The default subnet mask has a standard size. The custom subnet mask allows you to make subnets that are smaller or larger than the default.
255.0.0.0 for a class A correct. but why? well, 10.10.0.0 is a class a network and all class "a" networks use a default subnet mask of 255.0.0.0. the range of class "a" networks is 1-126.
The default subnet would be 255.0.0.0 (class A)
ip addresses from 172.16.0.0 thru 172.31.255.255 are private and do not route on the internet.
If this is a default subnet mask, then it would be a class C subnet mask. If you are subnetting a network and this is not the default subnet mask, then it could be either a class A or class B.
A router
There is no set subnet mask for different ip configurations. The most common default subnet mask is 255:255:255:0 though, if that helps.
255.0.0.0The default subnet mask of a class A network is 255.0.0.0