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Key points to understanding the Network Block
IP Address, Network Address and Broadcast Address are three very distinct numbers. These can only be determined if you know the net mask
The net mask is always a 32 bit number with the network portion being all 1's and the node portion being all 0's
Default Class A is 255.0.0.0 OR 8 Bits OR 1st Octet
Default Class B is 255.255.0.0 OR 16 Bits OR first 2 Octets
Default Class C is 255.255.255.0 OR 24 Bits OR first 3 Octets
A network is said to be sub netted when the mask is not the default mask
When a network is sub netted it always means there will be more networks and less nodes on each.
The Broadcast Address of any network is when the node portion of the 32 bit number is all 1's
For example 1: Network 192.168.1.0/24 Node portion is last octet, network is first 3 octets. Broadcast will be when last octet is all 1's - i.e. 192.168.1.255 For example 2: Network 192.168.1.0/26 Node portion is last 6 bits, there are 2 networks when we borrow 2 bits - .64 (01) and .128 (10) Broadcast addresses will be when the node portion is all 1's - i.e. 192.168.1.127 and 192.168.1.191 This is because when we see those two networks in binary and have all 1's in the node portion. 01[111111] = 127 10[111111] = 191
If we know the net mask in dotted decimal notation we can work out what has been borrowed
The key to any problem about sub netting is knowing how many bits borrowed and how many remain
If we have a network address and need to know how many bits need to be borrowed to subnet it down we use the Adding Trick (16 - 8 - 4 - 2 - 1) i.e. keep adding from the right until you have a number greater than the number of networks you need - That column is the number of High Order bits you need to borrow in the mask.
Networks = 2 to the power of bits borrowed - 2
Network Block = 2 to the power of bits remaining
Nodes on each network = The Network Block - 2