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CIDR format and borrowed bits

More than 6 bits (the last of the fourth octet) for the Subnet can not borrow because it will not be anything bits for hosts.

Well, at least we must remain two (2) Host bits in the fourth octet. (borrowing 6 bits for the network is mostly used for Point-to-Point connections, because we left two bits and Host 2^2=4-2=2 IP addresses to host 2

CIDR format IP address

If you have a good look at photos 11-15, you could notice and CIDR slash format writing subnet mask /25, /26, /27, /28, /29, /30 as well as the decimal notation Subnet Mask that is the last principle of Subnetting.

Let's get back to writing slash format Subnet mask. Why is this so?
I have already mentioned borrowing host bits. These borrowed bits add up the network bits (Network-N) from part of Network and the sum of the total number of bits have been written as a CIDR (Classless Inter-Domain Routing) format.

Thus, the default network mask 255.255.255.0 network has 24 bits (8+8+8+0)

If you borrow a certain number of bits we get the new network subnet mask.
24+1=/25 (one loan will total 2=subnet) is not advisable to borrow only 1 bit.

24+2=/26 (two borrowed bits = 4 total subnet)
24+3=/27 (three borrowed bits = 8 total subnet)
24+4=/28 (four borrowed bits = 16 total subnet)
24+5=/29 (five borrowed bits = 32 total subnet)
24+6=/30 (six borrowed bits = 64 total subnet)

To better understand refer to the table in Figure 16.

Figure 16.

Classless Inter-Domain Routing format IP address

If, you have the IP address of the Class B (remember the default mask = 255.255.0.0) and you want to perform subnetting, you can borrow 8 bits and you can borrow up to 12 bits (8 bits of the third octet, and 4 in the fourth octet)
Let's go to the next page tutorial.