The netmasks tell each machine which addresses can be reached directly (are on your local network) and which addresses require forwarding packets through a router. If the netmask is wrong, the machines will make one of two mistakes. One is to try to route local packets via a router, which is an expensive way to waste time - it may work reasonably fast, it may run slowly, or it may fail utterly. The second mistake is to fail to send packets for a remote machine to the router, which will prevent them from being forwarded to the remote machine.
The netmask is a number like an IP address, with one-bits for the network part of an address and zero-bits for the host portion. The netmask is literally used to mask off parts of the address inside the TCP/IP code. If the mask is 255.255.0.0, the first 2 bytes are the network part and the last 2 are the host part. More common is 255.255.255.0, in which the first 3 bytes are the network part and the last one is the host part.
For example, let's say your IP address is 192.168.0.10 and the Samba server is 192.168.236.86. If your netmask happens to be 255.255.255.0, the network part of the addresses is the first 3 bytes and the host part is the last byte. In this case, the network parts are different, and the machines are on different networks:
If your netmask happens to be 255.255.0.0, the network part is just the first two bytes. In this case, the network parts match and so the two machines are on the same network:
Of course, if your netmask says one thing and your network manager says another, the netmask is wrong.
