Hi I've been learning networks for a while now and I can't seem to find a solution for this doubt online.
I learnt that if you are sending a packet to a class B network's destination 141.49.255.255, it means you are broadcasting the packet to the whole class B network of 141.49.0.0; I also learnt that if you are sending a packet to a class A network's destination address like 10.255.255.255, you are again broadcasting the packet in the network ID 10.0.0.0
My question is, what am I doing when I'm sending a packet to class B 141.255.255.255 ? Am I sending it to a single host or a network?
Forget classes. Everything is being done in CIDR for the last two decades.
Sending to 141.255.255.255/16 is a subnet broadcast to 141.255.0.0/16. The same address may also be
141.255.255.255/8 - subnet broadcast to 141.0.0.0/8
141.255.255.255/6 - unicast(!) to the node 141.255.255.255
Most often, broadcasts are sent to 255.255.255.255. Routers (usually) don't forward broadcasts, so it's limited to the segment anyway.
However, there are some subtle differences: running e.g. 192.168.1.0/24 and 10.1.0.0/16 inside the very same Ethernet segment, a broadcast to 192.168.1.255/24 is physically sent to all nodes (Ethernet broadcast), even the ones with a 10.1.0.0/16 address. These however will the drop and ignore the packet since it doesn't address them.
In contrast, a broadcast to 255.255.255.255 will be sent to all nodes and processed by all nodes.
Related
Network disconnect issue happens in a system of my company.
Here is the network topolgy:
PC1: two NICs, both static IP address. Data: 10.10.22.11, Control: 10.10.22.10
PC2: two NICs, both static IP address. Data: 10.10.22.101, Control: 10.10.22.100
Default Gateway 10.10.22.11 is set on both side. However I don't think this is necessary, as there is no router or gateway between the two PCs, there directly linked.
A consultant point out, since all the IPs are set in same segment 10.10.22, there could be broadcast storm, which might be the cause of network disconnect.
Is this true? Can broadcast storm happen in directly linked two PCs?
No it shouldn't be.
Broadcast storms happen when there is a loop in a network.
A packet is forwarded to all ports in a switch and if there is a loop the packets are again sent to the same switch on all ports, amplifying the storm, if there is no network loop there shouldn't be any storm.
I don't see any loop in your configuration so this shouldn't be any broadcast storm.
Identifying broadcast storm is not so hard, just sniff on an interface on the network, and if you see millions of times the same broadcast packet, it should be a broadcast storm.
I am sorry if its basics, but I did not find the appealing answer for it over the Internet.
Why only MAC is used to transfer the packet to a device ? MAC address is only obtained by ARP for a specific IP address. So, why not just let the routers maintain IP addresses of the neighbouring routers and route packets using IP addresses of routers instead of MAC addresses ?
Why not redesign the architecture, to only use IP address for routing as well as moving the packet in the data link layer too ?
Why do we need MAC addresses?" Why can't network devices such as the routers just send the packet to the next router using the router's IP address?
Note : I know that MAC address is used to identify the system in a network. But you see the source never knew the MAC address of receiver. All it knew was its IP address and MAC address of next hop.
I'm reading Data Comm and Networking by Forouzan ( Ed 5) and it says that even routers have an IP address. So why use the mac address at all. The router can store the IP address of the source and route it to the next router .
EDIT : The question that I was getting as suggestion to this one does not answer my query. There are multiple counter points and proof that I have presented here which could have been done which is not answered by the one which is suggested. So please read my question before making any assumptions.
What do you think makes more sense: Having one protocol like Ethernet handle all the layer 2 details so that its layer 3 payload doesn't have to care, or force IP, ARP, WoL, IPX, MPLS, SLPP, and dozens more implement it on their own? The whole purpose of OSI layers is that upper layers need not know all the lower layer's details and lower layers need need not support the upper layer's features.
MAC addresses are used for the layer 2 protocol which encapsulates a layer 3 protocol. If all the necessary features were embedded into IP, then you'd be leaving other protocols to re-implement layer 2 routing on their own. This would be wildly inefficient.
I think at L2 it will be ether type and destination MAC address.
at L3 it can be protocol field and Destination IP Address.
I want to know what are the other fields by which we can differentiate between control and data packet.
#Anupam Thakur: In pure L2 infrastructure, on receiving frame, node does the bridge-table/l2-table/cam lookup based on destination mac address. For different BPDUs (almost always reserved multicast mac dst addresses, few exceptions beyond the scope of this question) wherever required cam table is programmed with punt path to control-plane module (/also). Note, punt path doesn't distinguish between different protocols. It's job is to just push the packet in Q towards control-plane module. When cp module receives such packet, based on ethertype or LLC content it determines the further protocol processing of the packet.
"Router Alert Option" in IP header can also force packet to be punted to Control Plane, even though it is not destined for recipient device. (i.e. transit node)
My question is pretty basic and my apologized for that. In TCP/IP network, if you take a look on layer 2 (Ethernet), we have some fields like Source and Destination MAC address. If you think in your browser, it is totally understandable that you can type a IP, and then the layer 3 (IP) will know the destination IP, then the frame is sent to layer 2.. However, how the layer two know the destination MAC address ? The source MAC address is stored in the network card and it is understandable but on what part of communication the destination MAC address will be known ?
I will give you the basic of how the routing works. This will clarify your doubts:
1) Assuming that we have a layer 3 IP packet which we want to send to some destination which have a DIP = y.
2) The route lookup in the routing table would give you the next hop and the outgoing interface through which the packet needs to go out.
3) Now we have the next hop. Still we dont have the layer 2 encap information which is the DMAC. Assuming this is the first packet we are sending and we dont have the DMAC vs next hop IP mapping yet.
4) The system will generate a ARP Query which will be broadcasted with a DMAC FF:FF:FF:FF:FF:FF within the subnet. This query asks who has the the next hop IP. (Remember we got this from the route lookup).
5) The router having the next hop IP would respond back to the src router (Unicast reply), thus we would know what is the DMAC that needs to be put on layer 2 header.
6) The L3 packet will now be encapsulated in layer 2 header with the dmac as already known now.
7) Remeber the dip won't change in the layer 3 header. This way the packet would traverse hop by hop. At each hop the layer 2 DMAC would change and not the DIP.
Remember that the MAC address identifies each router uniquely on a hop by hop basis.
Hope that clarifies your doubt.
Your computer determines the destination MAC address of the next hop is typically determined on an Ethernet network by using ARP, an Ethernet broadcast protocol that allows you to ask which device is assigned a particular IP address.
MAC address is resolved using layer-2 devices such as network switches. ARP tables is the tool to resolve IP to MAC.
# arp -an
I am familiar with the basic OSI model but I always get confused how does a packet travel from one machine to another over the Internet and what OSI layers do come into picture? For example, for the following topology:
Machine A<----->Switch<---->Router<---->Router<---->Router<---->Switch<---->Machine B
where the multiple routers are shown to represent the Internet, what happens at the OSI layer level, when Machine A send a packet (say a simple "ls" command over FTP) to Machine B.
The above is just a suggested example, but if any one can explain with any other network topology, that is fine too. All I am looking a very basic explanation of how the packet gets transformed to different OSI layers at each nodes (Machine, Switch, Router, etc.).
Routers use the IP layer (layer 3) and switches use the data-link layer (layer 2). Layer 1 is the physical 1s and 0s that go over a wire, Layer 2 is the data-link layer, which is protocols like Ethernet and Point-To-Point Protocol (PPP), which carries information between adjacent nodes about MAC address from and to and allows for error detection and retransmission. Layer 3 is the IP layer, which carries information about where in the whole network the packet is from and to, not just the current hop.
The transmission would go like this:
Machine A wants to send a packet to Machine B. Machine A knows Machine B's IP address, so it places that in the layer 3 packet. Machine A needs to place the MAC Address of the next hop in the layer 2 packet, however. If it does not know, then it will send something called an ARP request (Address Resolution Protocol, read here: http://www.tildefrugal.net/tech/arp.php ) to the network, with the destination IP. One of a few things will happen here:
The IP is local. The machine with that IP will reply back to the sender with its MAC address.
The IP is non-local. The gateway router will detect this and send its MAC address.
The IP is non-local and Machine A's default gateway and subnet mask are set. Using this information Machine A can determine the non-locality of the IP address and send it to the router's MAC address (ARPing if not known yet).
(If Machine A found this out earlier, it will be in the ARP cache and Machine A will just use that.) Now that the MAC address is sent, the packet can be transferred (the physical layer 1 performing the actual transfer of data on the wire). The next stop will be the switch. The switch knows which outbound port the MAC address listed as the layer 2 destination is on, because it tracks every MAC address it's seen a packet come from and which port it came on - if it does not know, then it will flood it out every single port, guaranteeing it'll arrive.
As such, the packet arrives at the router. The cool thing about the IP model is that it divides every single IP address in the network/world into a hierarchy - Subnets by definition cannot overlap subnets partially, they either wholly contain them or are wholly contained by them. So as long as subnets follow this hierarchy, the router can unambiguously determine where each of the 4 billion possible IP addresses are on the network just by looking at what subnet the IP will fall under in its table! The packet is then sent out that port.
As the packet travels through interconnected ISPs' routers, backbone infrastructure and so on, it arrives at Machine B's router, where the opposite process happens - router B sees that its destined for Machine B and sends it inbound. (Similarly, Router B will have to use a process like ARP to find Machine B's MAC address if not known.) The rest should be trivial from here.
good references:
https://web.archive.org/web/20120129120350/http://www.tildefrugal.net/tech/arp.php
http://en.wikipedia.org/wiki/Data_link_layer
http://en.wikipedia.org/wiki/Network_switch
http://en.wikipedia.org/wiki/Network_layer
http://en.wikipedia.org/wiki/Routing
http://en.wikipedia.org/wiki/Router_(computing)
http://en.wikipedia.org/wiki/Address_Resolution_Protocol
The only thing that can travel over a copper wire are pulses of electricity.
The binary number 1 is represented by a pulse of electricity or no pulse of electricity for 0.
Just keep in mind that real data of any kind cannot be sent over copper wire, fibre optic, or through the air ...only a representation of the data which has previously been converted to a 1 or a 0 and then is reconverted back at the receiving end.
Network layer protocol supervises the transmission of packets from a source machine to a destination. Data is broken down into packets, or datagrams, up to 64 kb long before it is transmitted, with a stamp of destination IP address, and forwarded to the network gateway. A gateway can be router to interconnect networks.