about broadcast ip

when we create subnet we subtract 2 from available hosts
one for network id
second for broadcast

as we use network ip in protocols like rip,eigrp,ospf

then where the broadcast ip is used

i am confused about this though done ccent

is this ip get converted into mac of broadcast i.e ff:ff:ff:ff:ff:ff

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Comments

okplaya

In order to have the routing protocol advertise a specific network you supply the network id. The routing protocol then advertises that network with the subnet mask. It finds out the subnet mask by checking the interfaces that are configured and/or depending on what you configured.

-DeXteR-

okplaya wrote: »

In order to have the routing protocol advertise a specific network you supply the network id. The routing protocol then advertises that network with the subnet mask. It finds out the subnet mask by checking the interfaces that are configured and/or depending on what you configured.

I Guess he was asking about the Broadcast id of a subnet , which is unusable in every subnet gonna be used for in routing protocols

MickQ

ccnaomkar wrote: »

when we create subnet we subtract 2 from available hosts
one for network id
second for broadcast

then where the broadcast ip is used

is this ip get converted into mac of broadcast i.e ff:ff:ff:ff:ff:ff

IP broadcast and MAC broadcast are 2 different things.

If two hosts are on the same subnet, they'll use MAC addressing to communicate, not IP addresses.

If they're on two different subnets, they'll use IP addressing to communicate (via the gateway).

for 192.168.1.0 255.255.255.0 (/24) subnet, the IP broadcast address is 192.168.1.255

for 192.168.1.0 255.255.255.128 (/25), the broadcast is 192.168.1.127

ccnaomkar

but what is use of broadcast id

jtdk985

ccnaomkar wrote: »

but what is use of broadcast id

im no expert (still studying for my ccna), but if you send data to a broadcast ip, it will send it to all ip's in that subnet range

MrRyte

ccnaomkar wrote: »

but what is use of broadcast id

(Taken from How a Broadcast Address Works : Learn-Networking.com )

Broadcasts are generally used for several reasons:

Address Resolution Protocol (ARP) uses broadcasts to map physical addresses to logical addresses. To build the table of hosts, a device needs to send a broadcast to every other device on the network to essentially find out who is where.
Several types of network protocols and services use broadcasts in the same way. Dynamic Host Configuration Protocol (DHCP), for instance, requires that broadcasts be used to dynamically assign an IP address to computers on a network.
Routing protocols such as Routing Information Protocol (RIP) use broadcasts to send out “advertisements.” This advertisement is used by routers to map out the topology of a network, so that data can be routed to the appropriate place accordingly. (Interesting enough, this protocol will attempt to find the fasted route through a network to a destination, based on how many “hops” it takes to get from the sender to the receiver.)

networker050184

It would be used for something referred to as a directed-broadcast. If a host sends a packet to the subnet 192.168.0.0/24 broadcast address of 192.168.0.255 the router on that subnet should replicate this to a local broadcast to all stations. Its basically a way for a device not on the local subnet to send a broadcast.

This isn't a desired behavior in todays networks though. I believe directed-broadcasts have been disabled by default in Cisco devices for quite some time.

huwwatkins

Isn't understanding broadcast/unicast/multicast one of the fundamentals of ccent?

networker050184

huwwatkins wrote: »

Isn't understanding broadcast/unicast/multicast one of the fundamentals of ccent?

It is, but from what I remember the material doesn't go into much detail, if any, into directed broadcasts. Its been a while since I've been through the CCNA stuff so I might be wrong.

ccnaomkar

networker050184 wrote: »

It is, but from what I remember the material doesn't go into much detail, if any, into directed broadcasts. Its been a while since I've been through the CCNA stuff so I might be wrong.

it is necessary to understand concept of unicast,multicast and broadcast

but unicast and multicast is easy as unicast to only one
multicast is to group of devices
and broadcast is to all devices on network

but where broadcast address is used i have seen debug on rip v1
it sends destination address as 255.255.255.255
then why it is not using 192.168.0.255 if network of 192.168.0.0 is used

if possible u can run it on gns3 or pt or lab
and post ur results if possible

okplaya

ccnaomkar wrote: »

but where broadcast address is used i have seen debug on rip v1
it sends destination address as 255.255.255.255
then why it is not using 192.168.0.255 if network of 192.168.0.0 is used

What if multiple networks are used (172.16, 10.10, etc)? The router(s) would need to keep track (overhead!) of every broadcast address for every network.

What about devices that don't know what network they're on yet (e.g. a PC booting up)? How could the PC know the specific network broadcast address?

From Wikipedia.....

"A special definition exists for the IP broadcast address 255.255.255.255. It is the broadcast address of the zero network or 0.0.0.0, which in Internet Protocol standards stands for this network, i.e. the local network"

instant000

ccnaomkar wrote: »

it is necessary to understand concept of unicast,multicast and broadcast

but unicast and multicast is easy as unicast to only one
multicast is to group of devices
and broadcast is to all devices on network

but where broadcast address is used i have seen debug on rip v1
it sends destination address as 255.255.255.255
then why it is not using 192.168.0.255 if network of 192.168.0.0 is used

if possible u can run it on gns3 or pt or lab
and post ur results if possible

that is EXPECTED behavior.

This is what you're referring to:

Cisco IOS Debug Command Reference, Release 12.3 - Debug Commands: debug ip msdp through debug ip scp [Cisco IOS Software Releases 12.3 Mainline] - Cisco Systems

Look for "debug ip rip" on this page, it shows what you're observing.

According to the RFC for RIP, this is the expected behavior:

Normally, requests are sent as
broadcasts

Source: RFC 1058 - Routing Information Protocol

I hope this helps.

Chris_

I think the OP is right to ask this question, we all blindly work out broadcast addresses as part of subnetting and think we know about them, but we probably don't.
Networked is bang on the money, I can't think of an example of a layer 3 network address being used apart from ip directed broadcast and no network engineer worth his salt is going to let them into his network!!!!!!
All the examples given ie ARP and DHCP would surely use the all F layer 2 broadcast address to go to all hosts within the layer 2 broadcast domain.

MickQ

Chris_ wrote: »

All the examples given ie ARP and DHCP would surely use the all F layer 2 broadcast address to go to all hosts within the layer 2 broadcast domain.

Yes, but don't forget RARP, inARP, BOOTP and DHCP.

ccnaomkar wrote: »

it is necessary to understand concept of unicast,multicast and broadcast

but unicast and multicast is easy as unicast to only one
multicast is to group of devices
and broadcast is to all devices on network

Don't forget that when a host sees that the frame/packet is addressed to that host, it will pass it up the stack and that costs processing time and resources. If it realises that the data is not actually meant for that host, then it gets dumped, but think about all the work that has been done in processing it to that point.

Unicast - processed by only the host specified.
Multicast - processed by the hosts listening out for that address, dumped by the others.
Broadcast - processed by EVERYONE. Oh think of all the resources wasted. Just imagine if you were in a room and everyone else in it spoke to you even though they wanted to say it to someone else.