WEBVTT 00:00:00.000 --> 00:00:06.000 align:middle line:84% The next type of Special Address is the Local Broadcast Address 00:00:06.000 --> 00:00:11.000 align:middle line:84% This is used to communicate with all devices on the local Network 00:00:11.000 --> 00:00:16.000 align:middle line:84% The address is populated with binary 1s so it looks like this 00:00:16.000 --> 00:00:19.000 align:middle line:84% in binary or this in decimal 00:00:19.000 --> 00:00:23.000 align:middle line:84% In other words all octets are populated with binary 1s 00:00:23.000 --> 00:00:27.000 align:middle line:84% which equates to 255.255.255.255 00:00:27.000 --> 00:00:31.000 align:middle line:84% this address is used for example by a host 00:00:31.000 --> 00:00:35.000 align:middle line:84% when requesting an IP Address from DHCP server 00:00:35.000 --> 00:00:39.000 align:middle line:84% The host doesn't have an IP Address yet, so it doesn't know which 00:00:39.000 --> 00:00:44.000 align:middle line:84% subnet it belongs to, it also doesn't know the IP Address of the 00:00:44.000 --> 00:00:50.000 align:middle line:84% DHCP server so it will send a broadcast to this address 00:00:50.000 --> 00:00:53.000 align:middle line:84% the Local Broadcast Address to request an IP Address 00:00:53.000 --> 00:00:57.000 align:middle line:84% from a DHCP server, just in case you're not familiar 00:00:57.000 --> 00:01:03.000 align:middle line:84% with DHCP, DHCP is Dynamic host Configuration Protocol 00:01:03.000 --> 00:01:06.000 align:middle line:84% and it's a service that provides IP Addresses dynamically 00:01:06.000 --> 00:01:12.000 align:middle line:84% to devices such as PCs, phones, iPads, IP Telephones 00:01:12.000 --> 00:01:16.000 align:middle line:84% and other devices that you may encounter on a Network 00:01:16.000 --> 00:01:20.000 align:middle line:84% So rather than you manually configuring IP Addresses 00:01:20.000 --> 00:01:26.000 align:middle line:84% on your iPhone or your iPad or an IP Telephone or a PC 00:01:26.000 --> 00:01:30.000 align:middle line:84% for that matter, you would allow that device to boot up 00:01:30.000 --> 00:01:34.000 align:middle line:84% sends out a broadcast to the Local Broadcast Address and the 00:01:34.000 --> 00:01:38.000 align:middle line:84% DHCP server hears that broadcast and then allocates 00:01:38.000 --> 00:01:43.000 align:middle line:84% an IP Address to the host from a pool of IP Addresses 00:01:43.000 --> 00:01:47.000 align:middle line:84% or scope of IP Addresses, you've probably encounter this 00:01:47.000 --> 00:01:52.000 align:middle line:84% many many times so as an example at home you may have a broadband 00:01:52.000 --> 00:01:57.000 align:middle line:84% or DSL type router providing wireless connectivity at home 00:01:57.000 --> 00:01:59.000 align:middle line:84% so that you can get on to the Internet 00:01:59.000 --> 00:02:03.000 align:middle line:84% when your PC or your iPad or other devices connect 00:02:03.000 --> 00:02:06.000 align:middle line:84% to that Wireless Network that automatically allocated 00:02:06.000 --> 00:02:10.000 align:middle line:84% an IP Address in the same way when you go to Starbucks 00:02:10.000 --> 00:02:14.000 align:middle line:84% or another coffee shop, you may be an IP Address 00:02:14.000 --> 00:02:18.000 align:middle line:84% via DHCP server providing both Wireless connectivity 00:02:18.000 --> 00:02:21.000 align:middle line:84% as well as IP Addresses to Client devices 00:02:21.000 --> 00:02:27.000 align:middle line:84% Be aware that Local Broadcast Addresses are always dropped 00:02:27.000 --> 00:02:31.000 align:middle line:84% by Layer 3 devices such as routers and Layer 3 switches 00:02:31.000 --> 00:02:35.000 align:middle line:84% you can override that functionality by configuring what 00:02:35.000 --> 00:02:40.000 align:middle line:84% is called DHCP forwarding also called DHCP relay 00:02:40.000 --> 00:02:45.000 align:middle line:84% on your routers and switches, so as an example if you had a PC 00:02:45.000 --> 00:02:49.000 align:middle line:84% or device in VLAN 10 but your DHCP server 00:02:49.000 --> 00:02:52.000 align:middle line:84% was on a different VLAN or different Network 00:02:52.000 --> 00:02:55.000 align:middle line:84% somewhere else in your topology 00:02:55.000 --> 00:03:00.000 align:middle line:84% the DHCP request from the PC or phone would not be received 00:03:00.000 --> 00:03:04.000 align:middle line:84% by the DHCP server by default, you would have to configure 00:03:04.000 --> 00:03:08.000 align:middle line:84% IP forwarding or IP DHCP relay 00:03:08.000 --> 00:03:13.000 align:middle line:84% on the switch or router to forward the DHCP request to the DHCP 00:03:13.000 --> 00:03:19.000 align:middle line:84% server, so you would type an example like ip helper address 00:03:19.000 --> 00:03:23.000 align:middle line:84% and then specify the DHCP servers IP Address on the router 00:03:23.000 --> 00:03:26.000 align:middle line:84% or Layer 3 switch, typically in this example 00:03:26.000 --> 00:03:30.000 align:middle line:84% the router or switch still blocks the Local Broadcast 00:03:30.000 --> 00:03:36.000 align:middle line:84% so when it receive that Local Broadcast from a PC or phone 00:03:36.000 --> 00:03:41.000 align:middle line:84% it drops it but then sends a Unicast DHCP request 00:03:41.000 --> 00:03:46.000 align:middle line:84% to the DHCP server on behalf of the PC or phone 00:03:46.000 --> 00:03:50.000 align:middle line:84% in VLAN 10, this then allows the router or switch to 00:03:50.000 --> 00:03:55.000 align:middle line:84% proxy the DHCP request on behalf of the PC or IP Phone 00:03:55.000 --> 00:03:59.000 align:middle line:84% What you need to be aware of is that traffic sent 00:03:59.000 --> 00:04:05.000 align:middle line:84% to this address 255.255.255.255 is dropped by routers 00:04:05.000 --> 00:04:10.000 align:middle line:84% in Layer 3 switches, additional configuration is required 00:04:10.000 --> 00:04:12.000 align:middle line:84% on the router or layer 3 switch if you want PCs 00:04:12.000 --> 00:04:18.000 align:middle line:84% in different VLANs to receive IP Addresses from a DHCP server 00:04:18.000 --> 00:04:21.000 align:middle line:84% in a remote or different VLAN and that's because once again 00:04:21.000 --> 00:04:24.000 align:middle line:84% and that's because once again routers and switches 00:04:24.000 --> 00:04:29.000 align:middle line:84% drop this Broadcast Address by default