Original subtitles

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 10

subnet it belongs to, it also doesn't know the IP Address of the 11

DHCP server so it will send a broadcast to this address 12

the Local Broadcast Address to request an IP Address 13

from a DHCP server, just in case you're not familiar 14

with DHCP, DHCP is Dynamic host Configuration Protocol 15

and it's a service that provides IP Addresses dynamically 16

to devices such as PCs, phones, iPads, IP Telephones 17

and other devices that you may encounter on a Network 18

So rather than you manually configuring IP Addresses 19

on your iPhone or your iPad or an IP Telephone or a PC 20

for that matter, you would allow that device to boot up 21

sends out a broadcast to the Local Broadcast Address and the 22

DHCP server hears that broadcast and then allocates 23

an IP Address to the host from a pool of IP Addresses 24

or scope of IP Addresses, you've probably encounter this 25

many many times so as an example at home you may have a broadband 26

or DSL type router providing wireless connectivity at home 27

so that you can get on to the Internet 28

when your PC or your iPad or other devices connect 29

to that Wireless Network that automatically allocated 30

an IP Address in the same way when you go to Starbucks 31

or another coffee shop, you may be an IP Address 32

via DHCP server providing both Wireless connectivity 33

as well as IP Addresses to Client devices 34

Be aware that Local Broadcast Addresses are always dropped 35

by Layer 3 devices such as routers and Layer 3 switches 36

you can override that functionality by configuring what 37

is called DHCP forwarding also called DHCP relay 38

on your routers and switches, so as an example if you had a PC 39

or device in VLAN 10 but your DHCP server 40

was on a different VLAN or different Network 41

somewhere else in your topology 42

the DHCP request from the PC or phone would not be received 43

by the DHCP server by default, you would have to configure 44

IP forwarding or IP DHCP relay 45

on the switch or router to forward the DHCP request to the DHCP 46

server, so you would type an example like ip helper address 47

and then specify the DHCP servers IP Address on the router 48

or Layer 3 switch, typically in this example 49

the router or switch still blocks the Local Broadcast 50

so when it receive that Local Broadcast from a PC or phone 51

it drops it but then sends a Unicast DHCP request 52

to the DHCP server on behalf of the PC or phone 53

in VLAN 10, this then allows the router or switch to 54

proxy the DHCP request on behalf of the PC or IP Phone 55

What you need to be aware of is that traffic sent 56

to this address 255.255.255.255 is dropped by routers 57

in Layer 3 switches, additional configuration is required 58

on the router or layer 3 switch if you want PCs 59

in different VLANs to receive IP Addresses from a DHCP server 60

in a remote or different VLAN and that's because once again 61

and that's because once again routers and switches 62

drop this Broadcast Address by default