How to know the broadcast address of your network

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In this post I am going to expalin about "how do we know the broadcast address of the network" ?


Example
Provide the broadcast address for the following:

IP Address: 221.240.224.121
Subnet mask: 255.255.255.248

Method I currently use.

2^Nth - 2 = number of usable subnets. 

2^5 - 2 = 30 subs

256 - last mask number = first network

256-248 = 8 or 221.240.224.8

Now add network range to itself equal to the total subnets calculated earlier to find all range start points. Stop when ip address falls in a range.

.8
.16
.24
...
.112
.120 (bingo)


Add range - 1 to start point to find broadcast.

120 + (8 - 1) = 127.


That is a pretty long process.

I do know of 2 sort cuts mainly divide till you find the range you want. Like start with 121/8 = 15.125 so to high try 120/8 =15 whole number so start of a range.

Also instead of adding subnet start with range - 1. Just go up a range and subtract
one.

120 jump to 128 subtract 1 = 127.

Anyone have any pointers to add or tricks? 


This is posted with the help of JacksRage

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