A subnet mask is a number that distinguishes the network address and the host address within an IP address1.
A subnet mask allows network traffic to understand IP addresses by splitting them into the network and host addresses. If the subnet mask is incorrect, the network traffic may not be able to determine the correct destination for the packets, and only reach some of the servers that have similar IP addresses. For example, if the new virtual server has an IP address of 192.168.1.100 and a subnet mask of 255.255.0.0, it can only communicate with servers that have IP addresses in the range of 192.168.0.0 to 192.168.255.2552. To fix this issue, the systems administrator needs to check and correct the subnet mask of the new virtual server according to the network configuration.