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The next type of Special Address is the Local Broadcast Address

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This is used to communicate with all devices on the local Network

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The address is populated with binary 1s so it looks like this

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in binary or this in decimal

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In other words all octets are populated with binary 1s

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which equates to 255.255.255.255

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this address is used for example by a host

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when requesting an IP Address from DHCP server

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The host doesn't have an IP Address yet, so it doesn't know which

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subnet it belongs to, it also doesn't know the IP Address of the

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DHCP server so it will send a broadcast to this address

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the Local Broadcast Address to request an IP Address

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from a DHCP server, just in case you're not familiar

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with DHCP, DHCP is Dynamic host Configuration Protocol

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and it's a service that provides IP Addresses dynamically

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to devices such as PCs, phones, iPads, IP Telephones

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and other devices that you may encounter on a Network

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So rather than you manually configuring IP Addresses

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on your iPhone or your iPad or an IP Telephone or a PC

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for that matter, you would allow that device to boot up

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sends out a broadcast to the Local Broadcast Address and the

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DHCP server hears that broadcast and then allocates

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an IP Address to the host from a pool of IP Addresses

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or scope of IP Addresses, you've probably encounter this

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many many times so as an example at home you may have a broadband

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or DSL type router providing wireless connectivity at home

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so that you can get on to the Internet

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when your PC or your iPad or other devices connect

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to that Wireless Network that automatically allocated

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an IP Address in the same way when you go to Starbucks

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or another coffee shop, you may be an IP Address

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via DHCP server providing both Wireless connectivity

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as well as IP Addresses to Client devices

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Be aware that Local Broadcast Addresses are always dropped

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by Layer 3 devices such as routers and Layer 3 switches

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you can override that functionality by configuring what

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is called DHCP forwarding also called DHCP relay

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on your routers and switches, so as an example if you had a PC

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or device in VLAN 10 but your DHCP server

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was on a different VLAN or different Network

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somewhere else in your topology

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the DHCP request from the PC or phone would not be received

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by the DHCP server by default, you would have to configure

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IP forwarding or IP DHCP relay

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on the switch or router to forward the DHCP request to the DHCP

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server, so you would type an example like ip helper address

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and then specify the DHCP servers IP Address on the router

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or Layer 3 switch, typically in this example

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the router or switch still blocks the Local Broadcast

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so when it receive that Local Broadcast from a PC or phone

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it drops it but then sends a Unicast DHCP request

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to the DHCP server on behalf of the PC or phone

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in VLAN 10, this then allows the router or switch to

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proxy the DHCP request on behalf of the PC or IP Phone

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What you need to be aware of is that traffic sent

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to this address 255.255.255.255 is dropped by routers

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in Layer 3 switches, additional configuration is required

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on the router or layer 3 switch if you want PCs

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in different VLANs to receive IP Addresses from a DHCP server

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in a remote or different VLAN and that's because once again

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and that's because once again routers and switches

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drop this Broadcast Address by default