NETWORK SECURITY

  • 04/12/2017
    7:30 AM
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IP Addresses, Subnet Masks, and Default Gateways

Learn the basics about how devices communicate in an IPv4 network.

Broadcasts

If the devices are in the same subnet, the mechanism used to determine the location of the destination device is the broadcast. A broadcast packet is a special packet on the subnetwork that's sent to every device on the subnetwork. The broadcast address is designated as 255.255.255.255. This is known as a limited broadcast because it is limited to the subnet that it originated from.

When a broadcast is received -- which is actually a broadcast MAC address that is local to that VLAN -- all devices within that subnet respond with their IP address and the physical MAC address of the network card. These two pieces of information are then placed into an address resolution protocol (ARP) table. The table is used to keep track of the location of devices on the same subnet. And ultimately, the ARP table is used to efficiently switch data on the same subnet at the data link layer of the OSI model.

As you can imagine, the larger the subnetwork is, the more broadcast and ARP traffic will consume your network. In our example, a 255.255.255.0 network mask can hold up to 254 different devices. The next largest network is 512, and then 1024. Depending on your network capacity, anything over 1024 hosts on a single subnet is going to create too much broadcast traffic. Instead, subnets are intended to shrink broadcast domains so that the broadcasts themselves have little to no impact on the performance of the network.

NEXT page: Default gateways

 

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