Add a single IP address or a subnet (a group of IP addresses) to the list of addresses granted or denied access to the WhatsUp Gold Central Site. The system is designed so that you can restrict access to everyone, and only put in the Allow list the networks (sites) that you want to grant access to the Central Site. If you only place addresses in the Allow list, everyone will be granted access.
If the IP address you entered is an IPv6 address (for example, 3ef0:3333:4444:5555::), you can only enter a prefix length in that field which is a number between 0 and 128.
If the IP address you entered is an IPv4 address (for example, 192.168.3.0), then you can specify either a mask (such as 255.255.255.0) or a prefix length which is a number between 0 and 32 for IPv4.
Note: Addresses which are neither on the allow nor the deny list are allowed. Addresses which are on both lists will also be allowed. Furthermore, the local host always has access, no matter what the settings are.
Prefix lengths and masks are equivalent. A prefix length indicates how many bits of the subnet IP address consist of the subnet prefix, or the number of bits of the masks that are set to 1.
For example, the subnet 192.168.3.0 255.255.255.0 has a prefix of 192.168.3. Its mask, 255.255.255.0, consists of 24bits set to 1 and 8 bits set to 0. Its prefix length is 24, which is often times written as 192.168.3.0/24.
The prefix length notation is sometimes referred to as the "CIDR" (Classless Inter-Domain Routing) notation. When a network does not follow the boundary of the classic class A, B, and C networks, the CIDR notation is often more readable than the classic mask one. For example, when the subnet 10.10.10.32 255.255.255.244 is written 10.10.10.32/27, using the prefix length of 27, you immediately know this subnet address space consists of the last 32-27=5 bits or 2^5=32 IP addresses between 10.10.10.32 and 10.10.10.63. IPv6 only supports the prefix length notation. For example, the subnet 2245:AB12:1987:1154::/64 is a subnet that consists of 2^64 IP addresses between 2245:AB12:1987:1154::0 and 2245:AB12:1987:1154:FFFF:FFFF:FFFF:FFFF.
Note: IPv6 is only available on IPv6-enabled computers.