By default, OpenVPN assumes a net30 topology for tun setups and a subnet topology for tap setups. With net30 on a tun setup, OpenVPN allocates a /30 CIDR network (four IPs, two usable)to each connecting client. In some cases, that is not desirable. The Topology Subnet option will make OpenVPN allocate only one IP per client, rather than an isolated subnet per client. This options is relevant only when supplying a virtual adapter IP address to clients when using tun mode on IPv4. Some clients may require this even for IPv6, such as OpenVPN Connect, though in reality IPv6 always runs with a subnet topology even when IPv4 uses net30. OpenVPN version 2.1.3 or newer is required to use this option, but there were significant fixes to it in OpenVPN 2.3 also, so using a current clientis important. Some clients may break if this options is used, such as older versions of OpenVPN,Windows versions with older tun/tap drivers, or clients such as Yealink phones. Because it is not universally supported, this option is off by default and we recommend leaving this option unchecked unless you are certain all possible clients for this server will support it.