It has got something to the with the Local network but I am not sure. Can anyone clear this doubt?
Essentially, in the usual IPv4 + Ethernet scenario there are three different kinds of broadcasts:
local IP broadcast to 255.255.255.255 - this broadcast reaches all nodes within the local broadcast domain; it is not forwarded by a router
directed IP subnet broadcast to the subnet address with all hosts bits set to "1", e.g. 192.168.1.255 for the 192.168.1.0/24 subnet - this broadcast reaches all nodes within the (possibly remote) subnet's broadcast domain; even though it is not forwarded by default on most routers, they are often configured to do so
Ethernet broadcast to FF:FF:FF:FF:FF:FF - this broadcast also reaches all nodes on the local broadcast domain and is the method of layer 2 transport used by the former two L3 broadcasts; a local IP broadcast directly translates to an Ethernet broadcast whereby a directed IP subnet broadcast is first routed to the destination network and then wrapped in an Ethernet broadcast by the last router
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While Layer-2 Broadcast is very simple and straightforward, have dst mac in ethernet hdr = Broadcast mac, and you are done (Eg. ARP broadcast request msg). My question is how to achieve Layer-3 Broadcast using TCP/IP stack.
What should be the content of ethernet hdr and IP hdr (in terms of src and dst mac and src and dst ip addresses)? Basically I am looking at what should be the content of packet headers to facilitate ip/layer-3 broadcast. I have a topology of L3 routers connected. I want to know how one L3 router could broadcast a frame/pkt so that it reaches every other router. How to do L3-broadcast?
Is dst ip in IP hdr should be 255.255.255.255 in addition to dst mac in erthernet hdr = FF:FF:FF:FF:FF:FF?
you need to send the packet to the subnet broadcast address. for example, you have a network 192.168.1.0 255.255.255.0.
If you are sending a packet to 192.168.1.255 (routed), the router will generate a broadcast with destination FF:FF:FF:FF:FF:FF in the ethernet header.
But, many router are suppressing this. On Cisco Router, you can enable this with the command "ip directed-broadcast" on a interface.
try it out in your LAN.. take the broadcast address of your subnet (the very last address) and make a ping to that address... you will see an answer from many devices (not all devices will respond, depending on their implementation)
WakeOn LAN for exaple take usage of this method.. google it.
its not possible to reach multiple L3 Network because there is nothing like "l3 Broadcast" broadcast is Ethernet! If you want to do it, you need a bridged network (l2circuit) between the 3 Sites. Or, you can use Multicast if you want a 1 to many stream.
edit: maybe i should mention, per definition of ip, there is no broadcast! ip knows unicast and multicast. so you will not find any one router in the world that has something implemented like ip-broadcast. you have to write your application based on multicast!
I do not understand one thing: does each device have a broadcast address? I am subnetting a network and I am giving each subnet one broadcast address and not each host. Is this correct?
For IPv4 networks, each subnet has a broadcast address. Whatever the highest address is for that particular subnet is the broadcast address.
Suppose I had a network at 203.0.113.0 with a netmask of 255.255.255.0. With this mask, hosts on this network can have addresses 203.0.113.1 through 203.0.113.254. Hosts on this network wishing to send a broadcast message to all hosts on the network would use 203.0.113.255.
I am new to IPs and subnets. I can calculate it and know the basics. Unfortunately, I couldn't find an answer to this question.
IPs ending on 0 designate the network address. Commonly, this address defines the route which traffic will be sent to.
Imagine that you have two routers connected by serial, so you have two subnets with 2 different private IPs: 10.0.0.0/8 and 11.0.0.0/8. /8 defines the address length in bits (Class A IP). First router will route traffic to 10.0.0.0 and second router will route traffic to 11.0.0.0.
IPs ending on 255 designate the broadcast address, a special address used to deliver packets to all hosts in the network.
For example, in case of ARP is very useful to locate/query other device's MAC by sending to the broadcast address a packet where you ask who has the MAC of a specefied IP. All the devices will capture your ARP request and if any of them has the specified IP, then you'll get a response with the MAC, but notice that you can be tricked since MAC can be impersonated (ARP Spoofing).
Regards.
I was studying about IP addresses when I came Across an address of type 127.0.0.1/8. I know that127.0.0.1 is a loop-back address but what did /8 mean in the address.what /8 (in 127.0.0.1/8) specify
the /8 in your ip is the subnet mask.
It allows you to know on which subnetwork you are, find your broadcast and unicast IP.
Take a look at http://en.wikipedia.org/wiki/Subnetwork to have more informations
/8 is a subnet mask written in CIDR notation and which is used by hosts to determine if they are on the same network as some other host and can communicate directly or they need to use a router (default gateway). In your case this is a loopback address, and actually all IP addresses from 127.0.0.0/8 network (from 127.0.0.0 to 127.255.255.255) are reserved loopback addresses as defined in rfc6890: Special-Purpose Address Registries
I am just preparing for a test in college about networking.
I'm currently trying around with sub netting and I found out that two devices attached to a layer 2 switch can talk to each other although they have different subnets!
Device A: 192.168.0.1 subnet mask : 255.255.255.0
Device B: 192.168.1.1 subnet mask : 255.255.255.0
The question is why I can ping from device A to B and vice versa?
There's no router, just the two devices and a switch.
From my understanding they should not see each other.
The OS should not even send the ARP request when the unknown IP is in a different subnet.
Could this be a caching issue?
Many Cisco Layer 2 switches are capable to ping the connected systems.
It is possible that your computer might have a route entry that sends a packet which matches no other specific route entry to your router. This is also called as the default gateway. Conventionally the computers in the same subnet are connected directly and most of the times, do not go through the gateway.
To explain it more clearly, If you're on a Linux machine, run route -n.
Destination Gateway Genmask
14.0.1.0 0.0.0.0 255.255.255.0
0.0.0.0 172.16.80.1 0.0.0.0
The first entry has a destination ranging from 14.0.1.0 to 14.0.1.255. The gateway for this match is 0.0.0.0. The table implies that these systems are connected directly. On the other hand, the entry 0.0.0.0 in the destination field will get matched when the packet matches none of the other entries. The gateway for this is the router address (which in my case in 172.16.80.1). All the packets that do not have IPs in the range that I specified above go to the router for further routing. Once the router gets the packet, it takes the further decision based on its routing information that it posses.
In your case the router happens to know that the other subnet is attached to it and hence passed your packet onto that subnet.
Have a closer look at the ARP. The ARP would be addressed to the router in your case of pinging to the other subnet. On the other hand, if pinging within the network, the ARP would be to the destination directly. This is the conventional case. Of course, it all depends on the routing tables in your system. You can always make all packets go through the gateway or no packets to go through the gateway.