All language subtitles for 8. Local Broadcast Address

The next type of Special Address is the Local Broadcast Address

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

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

in binary or this in decimal

In other words all octets are populated with binary 1s

which equates to 255.255.255.255

this address is used for example by a host

when requesting an IP Address from DHCP server

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