Subnetting (3)

Review

Subnetting Class A

  • 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)

Variable-Length Subnet Mask (VLSM)

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).

Steps

  • 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.

Resources

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