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Networking fundamentals: Subnetting

Subnetting is the process of breaking a network into multiple logical sub-networks. An IPv4 address is comprised of four octets of eight bits or thirty-two bits total. Each octet is converted to decimal and separated by a dot for example: 11111111.11111111.11111111.00000000 = 255.255.255.0

The Subnet Mask allows the host to compute the range of the network it's a part of, from network address to broadcast address. 

A device with an IP address of 192.168.1.5 with a subnet mask of 255.255.255.128 knows that the Network address is 192.168.1.0 and the Broadcast Address is 192.168.1.127.

Each place in the octet string represents a value:

128  64    32    16      8       4       2       1

1       1       1       1       1       1       1       1

When added together (128+64+32+16+8+4+2+1)= 255.

Network Class Ranges

Depending on what value is used, an IP represents a different class of network:

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Most LAN networks use private IP addresses outlined here:

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These addresses cannot be routed on the public Internet, but that is why the edge of the network will typically be using NAT (Network Address Translation) to translate the private IP addresses to public addresses. Using subnetting, one can split these private IP addresses to fit as many hosts as needed depending on the subnet mask that is used. The subnet mask divides the network portion (network bits) of the address from the host portion (host bits).

Typical Private Range Masks

Class A: 255.0.0.0

11111111.00000000.00000000.00000000

[-network-].[-----------------host---------------]

Class B: 255.255.0.0

11111111.11111111.00000000.00000000

[-------network--------].[---------host------------]

Class C: 255.255.255.0

11111111.11111111.11111111.00000000

[--------------network---------------].[---host---]

Cisco Meraki allows users to input subnet masks using CIDR notation which is an easier method of appending a subnet mask. If the subnet mask being used in a Class C network is 255.255.255.240, the CIDR notation would be /28 because the network portion (below in blue font) of the mask borrowed four bits from the host portion (red). The borrowed bits are in blue:

255.255.255.240 =11111111.11111111.11111111.11110000

Network Example

The Cisco Meraki MX appliance allows the administrator to configure subnets to their corresponding VLANs at Configure > Addressing & VLANs. Using VLANs help to logically separate subnet segments minimizing broadcast domains. For help setting up VLANs on the MX, please see here. VLANs will need to be recognized downstream on the LAN (The switch). The MX can handle routing between these VLANs (Inter-VALN Routing )so that different VLANs can still communicate if needed.

An example of VLANs with their corresponding Subnets and the IP on the MX that those VLANs point to:

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Below is an example of Per-Port VLAN configuration.

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Below is a network diagram to help outline the above configuration:

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Last modified
18:06, 7 Dec 2015

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