Hi All
I ve two PC's A and B which are connected to LAN1 .A is listening on UPNP multicast address 239.255.255.250 .
I have a mobile device D which is connected to a WIFI and the WIFI is connected to LAN1 .
Ip address of PC A is xxx.yyy.209.80 PC B is xxx.yyy.209.81 and device D is xxx.yyy.209.82.
If I send a multicast packet to 239.255.255.250 from PC B it reaches PC A.
But if I send a multicast pakcet from device D it never reaches PC A .Can anyone tell me why ?How to resolve it ?
Thanks in advance
Kozlov
I realize this post is 3 years old but the following information may be useful to someone. I have an iPhone App that uses a 239.x.x.x address and I have issues with certain wireless access points. I found a document that says the following:
There are some special multicast groups, say "well known multicast
groups", you should not use in your particular applications due the
special purpose they are destined to:
224.0.0.1 is the all-hosts group. If you ping that group, all multicast capable hosts on the network should answer, as every
multicast capable host must join that group at start-up on all it's
multicast capable interfaces.
224.0.0.2 is the all-routers group. All multicast routers must join that group on all it's multicast capable interfaces.
224.0.0.4 is the all DVMRP routers, 224.0.0.5 the all OSPF routers, 224.0.013 the all PIM routers, etc. All this special multicast groups are regularly published in the "Assigned Numbers" RFC.
In any case, range 224.0.0.0 through 224.0.0.255 is reserved for local
purposes (as administrative and maintenance tasks) and datagrams
destined to them are never forwarded by multicast routers. Similarly,
the range 239.0.0.0 to 239.255.255.255 has been reserved for
"administrative scoping" (see section 2.3.1 for information on
administrative scoping).
Anyway I changed my address to 238.x.x.x and that did not help with the Netgear R6100 AP. The issue I have seen with Wireshark is that LAN packets are not forwarded to wireless devices. I have not tried the other direction.
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.
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.
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.
I have a dumb question, first of all sorry for that. i am learning now 7 OSI Layer models and i stumble across one thing. The Ethernet which is in the second Data Link Layer provides the end-to-end connection via LANs, right? Does it mean that even if i connect to Internet thru WiFi, somewhere my connection is running thru LANs?
Thanks
I am going to break my answer into points:
1.Lan is not necessary to connect to internet. You can have cable internet or internet through DSL connection etc. in which though the wire that comes and connect to your pc is the same cat5(for example) cable, there is no lan involved.
2.Internet through wireless router an be accessed in two ways:
(a)The ethernet cable from (say) a cable modem connects to your wireless router WAN port and then wireless router sends out radio waves which your laptop catches.
(b)There are many wireless routers and one cable internet connection. So the WAN port of one of the routers will connect to the cable modem(same as (a)) and from that router's ethernet ports(usually there are 4 ethernet ports on a wireless router ), ethernet cables will connect to other routers' ethernet ports and now these routers will send out radio waves which your pc catches.
3.Ethernet is one of the IEEE protocols(IEEE 802.3) for the data link layer. The wifi uses IEEE 802.11 standards to communicate through wireless media.
Short answer is: most likely
In your case, you only know your direct connectivity is made possible thru WiFi. From your perspective, it's just a WiFi network. But behind WiFi network, it could be Ethernet, DSL, Cable, etc. And behind those, it could be T1, frame relay, ATM, 10G or maybe 100G Ethernet, etc.
For example, I can have a small LAN at my home while my company office building can have a much bigger enterprise LAN. And both can provide me with the same kind of WiFi access.
Your connection to the nearest router is using a wifi data link protocol (in the IEEE 802.11 family). But the connections to other routers and (eventually) hosts will use other data link protocols, likely including ethernet at least at the far end.
I'm having trouble understanding how multicast addresses work.
First off, is it true that if I have N clients or peers working on separate networks, they all subscribe to the same multicast group, and this group allows any source to send messages, these peers/hosts can all communicate to each other through this group? (sounds like black magic!)
Second, I've heard hints that the IANA controls/regulates the multicast addresses. So do you have to request / ask IANA for a specific multicast address for your project / company? How does this work? Am I mistaken? Can you clarify multicast networking for me?
Thanks much!
Firstly, multicast only works if the routers in the network support it. The IGMP protocol is used for this purpose: http://en.wikipedia.org/wiki/Internet_Group_Management_Protocol. Assuming you have N clients on separate networks connected by multicast capable routers, then they could send messages to a multicast address and other peers that have subscribed to that multicast address will then receive those messages. You can read up further on multicast on Wikipedia.
I think you're confusing IEEE with IANA, the Internet Assigned Numbers Authority (IANA). Have a look at http://www.iana.org/assignments/multicast-addresses/multicast-addresses.xml for multicast address assignments.