how to find a subnet mask and network address give an ip o 172.16.117.77/20?

Given Info:

- IP address: 172.16.117.77

- Subnet Mask: /20

1. Convert the subnet mask to it's decimal format. A /20 means there are 20 1's in the mask: 1111 1111.1111 1111.1111 0000.0000 0000 -- or 255.255.240.0. So a /20 gives us a '240' in octet-3. Remember that.

2. Before we go too far, let's figure out what the default subnet for our given IP would *normally* be if we weren't changing it. An IP starting with "172" makes it a Class B IP -- which means a default subnet mask of 255.255.0.0 or /16, if you will. We, however, have been told to use a /20, which means we're using 4 more 1's than the default mask.

3. So here's our new mask: 1111 1111.1111 1111.ssss hhhh.hhhh hhhh -- where the 1's are the network bits; the S's are the subnet bits; and the H's are the host bits. Using 4 subnet bits means we can use 16 different networks (2^4) -- with those network numbers using octet-3. Using 12 host bits means we can handle 2^12(-2) hosts or 4096-2 hosts/network. Let's list the networks:

- 172.16.0.0 -- The first Network. Add '16' to octet-3 to get the start of the next network.

- 172.16.16.0

- 172.16.32.0

- 172.16.48.0

- 172.16.64.0

- 172.16.80.0

- 172.16.96.0

- 172.16.112.0 -- your IP is in this range

- 172.16.128.0

- 172.16.144.0

- 172.16.160.0

- 172.16.176.0

- 172.16.192.0

- 172.16.208.0

- 172.16.224.0

- 172.16.240.0

Let's expand the 112 subnet:

- 172.16.112.0 -- Network Address - Unusable

- 172.16.112.1 -- 1st Usable IP

- 172.16.112.2 -- 2nd Usable IP

...

- 172.16.117.77 -- Your starting IP

...

- 172.16.127.254 -- Last Usable IP

- 172.16.127.255 -- Broadcast IP - Unusable

- 172.16.128.0 -- Next Network Address

...

20 Subnet Mask