255 is the largest value that can be represented with 8 bits, with one octet. 11111111 = 255 2 10 The subnet mask is always one of the following numbers: 254, 252, 248, 240, 224, or 192 To see why, look at the following: 255 11111111 255 11111111 255 11111111 255 11111111 255 11111111 -1 -1 -3 -11 -7 -111 -15 -1111 -31 -11111 --- -------- --- -------- --- -------- --- -------- --- -------- 254 11111110 252 11111100 248 11111000 240 11110000 224 11100000 255 11111111 255 11111111 -63 -111111 -127 -1111111 --- -------- --- -------- 192 11000000 128 10000000 The above numbers (subnet masks) go from one zero to two zeros to three zeros to four zeros to five zeros to six zeros and finally to seven zeros. ------------------------------------------------------------------------------ C:\>ipconfig Windows NT IP Configuration Ethernet adapter SMCISA1: IP Address. . . . . . . . . : 134.161.243.85 Subnet Mask . . . . . . . . : 255.255.252.0 Default Gateway . . . . . . : 134.161.240.1 Here is output of the ipconfig command when the /all switch is included. C:\>ipconfig /all Windows NT IP Configuration Host Name . . . . . . . . . : mark-jacobson.uni.edu DNS Servers . . . . . . . . : 134.161.1.32 132.161.1.11 Node Type . . . . . . . . . : Hybrid NetBIOS Scope ID. . . . . . : IP Routing Enabled. . . . . : No WINS Proxy Enabled. . . . . : No NetBIOS Resolution Uses DNS : No Ethernet adapter SMCISA1: Description . . . . . . . . : SMC Adapter. Physical Address. . . . . . : 00-00-C0-EF-BD-42 DHCP Enabled. . . . . . . . : Yes IP Address. . . . . . . . . : 134.161.243.85 Subnet Mask . . . . . . . . : 255.255.252.0 Default Gateway . . . . . . : 134.161.240.1 DHCP Server . . . . . . . . : 134.161.1.16 Primary WINS Server . . . . : 134.161.224.2 Secondary WINS Server . . . : 134.161.224.5 Lease Obtained. . . . . . . : Tuesday, January 12, 1999 7:32:06 AM