CCNA Subnetting in 5 Minutes

Subnetting is about the subnet mask and has much less to do with the IP address. The purpose of subnetting is to conserve IPv4 address space.

Consider for example a branch office where multiple subnets are often assigned based on traffic type and each subnet has a network address. The purpose is to segment traffic for performance and management purposes. Routers examine inbound packets and forward them between subnets.

Network Topology

192.168.1.0/24 = user VLAN

192.168.2.0/24 = wireless VLAN

192.168.3.0/24 = guest VLAN

You have to create subnets when implementing network addressing however subnetting further subdivides a single network address into multiple subnet addresses.

The IP address 192.168.1.0 is a class C address and has a default subnet mask of 255.255.255.0 (/24).

Subnetting is enabled when a non-default subnet mask is configured for any address class.

Instead of using three network addresses why not use one network address. Subnetting borrows bits from the host portion bolded and assigns them for subnet (network) addresses.

192.168.1.0 = 11000000.10101000.00000001.00000000

Example:

Create five subnets from IP address 192.168.1.0 so that each will have a minimum of 10 hosts addresses that are assignable per subnet address.

Number of Hosts Calculation

4 rightmost bits = 2 power 4 = 16–2 (for network and broadcast address) = 14 host addresses available.

why not 3 rightmost bits?

2 power 3 = 8–2 (for network and broadcast address) = 6 hosts addresses so not enough.

so now only 4 bits are assigned to hosts (bolded) instead of 8 bits, and there are 4 bits that are available for subnetting from the 4th octet (4 leftmost bits of 4th octet).

192.168.1.0 = 11000000.10101000.00000001.0000 0000

Number of Subnets Calculation

4 bits = 2 power 4 = 16 subnets are now available with a maximum 14 hosts per subnet. The 4 bits assigned for subnetting from the 4th octet is bolded.

192.168.1.0 = 11000000.10101000.00000001.00000000

new subnet mask = 255.255.255.255.240 (/28)

255.255.255.240 = 11111111.11111111.11111111.11110000

or 32 bits (IPv4 address) — 4 host bits = 28 network bits

You have now created 16 different subnets from one network address that are assignable to 16 different VLANs for example.

255.255.255.240 = 11111111.11111111.11111111.11110000

16 subnets = (0, 16,32,48,64,80,96,112,128,144,160,176,192,208,224,240)

Subnet 1 (zero subnet)

network address = 192.168.1.0/28

host range = 192.168.1.1/28–192.168.1.14/28

broadcast address = 192.168.1.15/28

Subnet 2 (.16 subnet)

network address = 192.168.1.16/28

host range = 192.168.1.17/28–192.168.1.30/28

broadcast address = 192.168.1.31/28

Subnet 3 (.32 subnet)

network address = 192.168.1.32/28

host range = 192.168.1.33/28–192.168.1.46/28

broadcast address = 192.168.1.47/28

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Subnet 16 (.240 last subnet)

network address = 192.168.1.240/28

host range = 192.168.1.241/28–192.168.1.254/28

broadcast address = 192.168.1.255/28

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