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!
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I am trying to understand the functioning of the different layers in the TCP/IP stack, and I just wanted some clarification on how the link layer derives the MAC address of NICs to receive packets.
This isn't a function of TCP/IP, per se. Instead, the ARP (Address Resolution Protocol) is used in IPv4 to translate the destination IP address to the correct MAC address.
This is accomplished by the stack by first determining if the delivery is local (within the subnet) by comparing the destination to the configured network mask. If it is local, ARP will be used to generate broadcast frames at the link layer, attempting to resolve the known IP address to the known MAC address.
On the other hand, if the destination IP address is determined not to be on the local subnet, the ARP protocol will be used to send a broadcast ARP at the link layer to discover the MAC address of the router that should be used based on the configured routing table.
Using IPv6, ARP is eliminated and replaced with multicast (more specifically, solicited node multicast) using the Neighbor Discovery Protocol over ICMP6.
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 want to trace packets as they move from my host to the destination - something like 'traceroute'. However, I need to know what to what port the packets are accepted at each of the network nodes while 'traceroute' gives me only the node.
Is this possible ? What tools should I use ?
By network nodes, I assume you mean router in the path. The question makes no sense since ports are transport layer addresses, but routing happens with network layer addresses.
When network traffic is routed, a router will strip off the layer-2 frame and inspect the layer-3 destination address (IP, IPX, etc.) in the layer-3 packet. The router never sees the layer-4 segment, so it knows nothing about the layer-4 address (TCP, UPD, SPX, etc. port). The router will switch the layer-3 packet to the next interface and encapsulate it in a new layer-2 frame for that interface, before sending it on its way.
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.
I connected two machines via a network cable. I need to get an ARP request data via Wireshark. When I pinged the IP of the other machine, I get the ARP request on Wireshark. But, it is not broadcasting a message. It targets pinged IP address directly. I think a LAN with only two machines does not need to do a broadcast. Am I right? Can any one explain this to me?
Always in ARP packet, MAC address will be broadcast not IP. As it is used to learn MAC address of other host whose IP address is known, ARP packet needs to have valid IP address rather than broadcast IP. You can check ARP packet example at below path:
http://wiki.wireshark.org/AddressResolutionProtocol
Hope this clears your doubt.