CCENT - Troubleshooting - IP Interface Question

Deborah Aretakis

When using the show IP Interface command, why is the broadcast address listed as 255.255.255.255 for an interface with an Internet address of 172.16.5.1/30 ? Wouldn't the broadcast address be 172.16.5.3 ? (I am working on the troubleshooting sections of the Premium lab for CCENT.) This is confusing. Please clarify. Thanks!

Ronnie Wong

@Deborah-Aretakis,

Great Question!
You're correct that for that SUBNET, the broadcast address is 172.16.5.3. This is called a directed broadcast and the limitation here is to every device on that same subnet. This type of broadcast is used by hosts that may be part of the subnet but many times is not. If I needed to deliver a broadcast message from a host from a different subnet then this directed broadcast would be used.

What you're seeing there is called a limited broadcast. This is a broadcast to every thing on the same physical LAN but not to the entire internet. According to RFC 1812, this is the preferred method. It's not that intuitive. You would think that with 255.255.255.255 you'd broadcast to every device on the internet but it essentially limited by devices that are Layer 2 connected. It does make sense though, if you convert that 255.255.255.255 to Hex you're sending to FFFF.FFFF.FFFF, this is the Layer 2 broadcast address. So it's not limited to the Layer 3 subnet.

Daniel Espinal

@Ronnie-Wong said in CCENT - Troubleshooting - IP Interface Question:

if you convert that 255.255.255.255 to Hex you're sending to FFFF.FFFF.FFFF, this is the Layer 2 broadcast address. So it's not limited to the Layer 3 subnet.

not to be a jerk but 255.255.255.255 is a 32 bit number while FFFF:FFFF:FFFF is a 48 bit number. You can't really convert between them because of the structure of the two addresses anyway. IP addresses have 4 sets of 8 bit numbers while MACs have 3 sets of 16 bit numbers. 255.255.255.255 actually translates into FF-FF-FF-FF.

This is a great question, I never realized there are different types of broadcasts. Learn something new everyday.

Furthermore the reason why it's better to use the IP of 255.255.255.255 is that it's limited to the LAN as Ronnie pointed out. By default directed Broadcast are disabled because if not then anyone not on your LAN can send a broadcast to your local LAN. This is very dangerous.

Ronnie Wong

@Daniel-Espinal said in CCENT - Troubleshooting - IP Interface Question:

@Ronnie-Wong said in CCENT - Troubleshooting - IP Interface Question:

if you convert that 255.255.255.255 to Hex you're sending to FFFF.FFFF.FFFF, this is the Layer 2 broadcast address. So it's not limited to the Layer 3 subnet.

not to be a jerk but 255.255.255.255 is a 32 bit number while FFFF:FFFF:FFFF is a 48 bit number. You can't really convert between them because of the structure of the two addresses anyway. IP addresses have 4 sets of 8 bit numbers while MACs have 3 sets of 16 bit numbers. 255.255.255.255 actually translates into FF-FF-FF-FF.

No doubt, as usual my poor attempt at relaying a concept failed. I just meant to say they're equivalent. the 32 bit subnet mask translates into the layer 2 broadcast of ALL bits.

This is a great question, I never realized there are different types of broadcasts. Learn something new everyday.

This is why the depth of networking protocols are defined in the RFCs. We normally don't need to go this far in depth in practical networking skills but it's nice to know that there is a final authority and a finite and clearly defined limits to networking.

Furthermore the reason why it's better to use the IP of 255.255.255.255 is that it's limited to the LAN as Ronnie pointed out. By default directed Broadcast are disabled because if not then anyone not on your LAN can send a broadcast to your local LAN. This is very dangerous.