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
subnet it belongs to, it also doesn't know the IP Address of the
DHCP server so it will send a broadcast to this address
the Local Broadcast Address to request an IP Address
from a DHCP server, just in case you're not familiar
with DHCP, DHCP is Dynamic host Configuration Protocol
and it's a service that provides IP Addresses dynamically
to devices such as PCs, phones, iPads, IP Telephones
and other devices that you may encounter on a Network
So rather than you manually configuring IP Addresses
on your iPhone or your iPad or an IP Telephone or a PC
for that matter, you would allow that device to boot up
sends out a broadcast to the Local Broadcast Address and the
DHCP server hears that broadcast and then allocates
an IP Address to the host from a pool of IP Addresses
or scope of IP Addresses, you've probably encounter this
many many times so as an example at home you may have a broadband
or DSL type router providing wireless connectivity at home
so that you can get on to the Internet
when your PC or your iPad or other devices connect
to that Wireless Network that automatically allocated
an IP Address in the same way when you go to Starbucks
or another coffee shop, you may be an IP Address
via DHCP server providing both Wireless connectivity
as well as IP Addresses to Client devices
Be aware that Local Broadcast Addresses are always dropped
by Layer 3 devices such as routers and Layer 3 switches
you can override that functionality by configuring what
is called DHCP forwarding also called DHCP relay
on your routers and switches, so as an example if you had a PC
or device in VLAN 10 but your DHCP server
was on a different VLAN or different Network
somewhere else in your topology
the DHCP request from the PC or phone would not be received
by the DHCP server by default, you would have to configure
IP forwarding or IP DHCP relay
on the switch or router to forward the DHCP request to the DHCP
server, so you would type an example like ip helper address
and then specify the DHCP servers IP Address on the router
or Layer 3 switch, typically in this example
the router or switch still blocks the Local Broadcast
so when it receive that Local Broadcast from a PC or phone
it drops it but then sends a Unicast DHCP request
to the DHCP server on behalf of the PC or phone
in VLAN 10, this then allows the router or switch to
proxy the DHCP request on behalf of the PC or IP Phone
What you need to be aware of is that traffic sent
to this address 255.255.255.255 is dropped by routers
in Layer 3 switches, additional configuration is required
on the router or layer 3 switch if you want PCs
in different VLANs to receive IP Addresses from a DHCP server
in a remote or different VLAN and that's because once again
and that's because once again routers and switches
drop this Broadcast Address by default