Subnetting (3)
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Last updated
10.0.0.0/8 network. 2000 subnets. Find the prefix length and the no. of hosts per subnet.
Borrowing 11 bits allows us to make 2000 subnets (2^11 = 2048).
So /19 prefix (8+11).
13 hosts remaining, so 8190 hosts per subnet.
Each bit you borrow doubles the number of subnets you can make.
10.217.182.223/11
Network addr : 10.192.0.0/11 (change hosts bits to 0)
Broadcast addr : 10.223.255.255 (change hosts bits to 1)
First usable addr : 10.192.0.1/11 (network addr + 1)
Last usable addr : 10.223.255.254 (broadcast addr - 1)
No. of hosts : 2,097,150 (21 host bits = 2^21 - 2)
Process of creating subnets of diff. sizes to make use of network addr. more efficiently.
Till now we used Fixed-Length Subnet Mask (FLSM) - all subets use the same prefix length (subnetting a class C network into 4 subnets using /26).
Assign the largest subnet at the start of the address space.
Assign the second-largest subnet after it.
Repeat the process until all the subnets have been assigned.
Tokyo LAN A
Network addr : 192.168.1.0/25
Broadcast addr : 192.168.1.127/25
First usable addr : 192.168.1.1/25
Last usable addr : 192.168.1.126/25
No. of hosts : 126 takes half of the total subnets
Toronto LAN B
Network addr : 192.168.1.128 (Tokyo A broadcast + 1)
Prefix length : /26 (because 45 hosts)
Broadcast addr : 192.168.1.191/26
First usable addr : 192.168.1.129/26
Last usable addr : 192.168.1.190/26
No. of hosts : 62 takes quarter of the total subnets
Toronto LAN A
Network addr : 192.168.1.192 (Toronto B broadcast + 1)
Prefix length : /27 (because 29 hosts)
Broadcast addr : 192.168.1.223/27
First usable addr : 192.168.1.193/27
Last usable addr : 192.168.1.222/27
No. of hosts : 30
Tokyo LAN B
Network addr : 192.198.1.224 (Toronto A broadcast + 1)
Prefix length : /28 (since 8 hosts, cannot use /29 because only 6 hosts are allowed)
Broadcast addr : 192.198.1.239/28
First usable addr : 192.198.1.225/28
Last usable addr : 192.198.1.238/28
No. of hosts : 14
Point-to-point connection
Network addr : 192.198.1.240
Prefix length : /30
Broadcast addr : 192.168.1.243/30
First usable addr : 192.168.1.241/30
Last usable addr : 192.168.1.242/30
No. of hosts : 2
In CCNA exam, don't use /31 prefix length for point-to-point connections.