The IP address 172.21.255.60 : Class B networks
Secure IP address This IP is only local, it is not accessible from outside the network
Rating: 5
This means that the IP address is not accessible from the Internet, it is local and only accessible within the network. Everyone has the same local address (172.16.0.0 - 172.31.255.255).
Each device (laptop, printer, scanner, phone, surveillance camera, tablet, ...) has a unique local IP address assigned by the adsl box that allows the box to identify the different devices to within a local network and send the right product at the right flow.
Specifically, the adsl box has a public IP address in your case, which connects to the Internet and the box assigns a local address to all devices to identify them.
The IP address 172.21.255.60 detail
The IP address 172.21.255.60 (IPv4 Class B networks) belonging to the network 172.16.0.0 - 172.31.255.255 is written in long version -1407844548.
Color obtained mathematically from the IP address: Bright violet
Addresses on the same network : 172.21.255.0 172.21.255.1 172.21.255.2 172.21.255.3 172.21.255.4 172.21.255.5 172.21.255.6 172.21.255.7 172.21.255.8 172.21.255.9 172.21.255.10 172.21.255.20 172.21.255.30 172.21.255.40 172.21.255.50 172.21.255.70 172.21.255.80 172.21.255.90 172.21.255.100 172.21.255.200 172.21.255.254 172.21.255.255