Q. How do I find out subnet mask on UNIX operating system? Can you tell us the name of command and configuration file to store subnetmask under UNIX?

A. A subnetwork/ subnet is a range of logical addresses within the address space that is assigned to an organization. Subnetting is a hierarchical partitioning of the network address space of an organization (and of the network nodes of an autonomous system) into several subnets. Routers constitute borders between subnets. Communication to and from a subnet is mediated by one specific port of one specific router, at least momentarily.

Under UNIX / Linux, you need to use the ifconfig utility to assign an address to a network interface
and/or configure network interface parameters. Same command can display subnet. Just type command ifconfig:
$ ifconfig
OR
$ /sbin/ifconfig
OR
$ /sbin/ifconfig interface-name

lnc0: flags=108843 mtu 1500
        inet 72.122.61.51 netmask 0xfffffff8 broadcast 72.122.61.55
        ether 00:0c:29:32:8a:8c
plip0: flags=108810 mtu 1500
lo0: flags=8049 mtu 16384
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 
        inet6 ::1 prefixlen 128 
        inet 127.0.0.1 netmask 0xff000000 

Look for inet line you will see an IP address, followed by netmask.