Private IP Address 172.21.0.4
172.21.0.4 belongs to the private IP address space 172.16.0.0 - 172.31.255.255 ( 172.16.0.0/12 ).
A private IP address is, in contrast to a public IP address, not allocated to a specific organization, but can be used by anyone without approval from a regional Internet registry. Private IP addresses can only be reached within a closed network and are not directly accessible from the outer Internet. On the one side this is considered a security enhancement because an external Internet host cannot directly communicate with an internal host. On the other side this works again IPv4 address depletion as many companies and individuals only need a single public IP address from their ISP, and still can access the Internet with more than one computer, smartphone, or other Internet connected devices simultaneously. To find out what external IP address your router is using please go to the My IP page.
IANA has allocated the subnet 172.16.0.0/12 for private use in February 1996 as noted in RFC 1918. It's a series of 16 contiguous class B networks with a prefix length of 16 bits each and spans 1,048,576 ( 16 * 216 ) IP addresses ranging from 172.16.0.0 to 172.31.255.255. However it is not uncommon to use only a part of the available address space for a private network, for example the smaller subnet 172.16.0.0/24 with 256 hosts. If you want to learn more about IP subnetting we encourage you to take a look at our subnet calculator.
The address 172.21.0.4 is sometimes misspelled, and you might see typical typing errors like these: 172.21.o.4
Network routers usually offer a configuration interface that is accessible via web browser on a URL like http://172.21.0.4 or https://172.21.0.4 or through a router specific host name alias.
See also: IPv4 List - Page 137,137
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