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Suppose we have messenger like GTalk, Skype etc running on two different PCs(A and B) which have private IP addresses but connected to Internet through some ISP. The two PCs could be located in different parts of the world. Now its possible to send messages and make VoIP calls between A and B.
This obviously uses server for communication establishment. For text messages it could be routed through the server to reach A and B.
But for VoIP calls, I believe initial establishment would be taking help of server and later VoIP data would flow directly between A and B. Now, A and B both have private IP addresses.
So my question is how is it possible to communicate between A and B directly over the Internet who have private IP addresses ?
Is there any specific standard for this ? I am not talking about VoIP standard like RTP etc but the mechanism to communicate over Internet using private IP addresses. In other words, I simply have a TCP client and TCP server running on A and B respectively. How can TCP client reach TCP server ?
I would like to know if messengers like GTalk, Yahoo, Skype and many of the Android applications like Viber, Kakoa talk, WhatsApp etc use any specific standard for this. And I have seen that these work almost always, calls between A and B work, located anywhere in the world.
It would be nice if somebody can share the architecture and the concepts involved.
[I did read about concepts like STUN and hole punching. But I don't see any specific way. And there is no guarantee that these would work depending on the NAT behavior]
Thank you.
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So, i was reading networking in depth. One concept i failed to understand was how different machines in LAN manage to access internet through one public IP address provided to company by ISP.
For eg. let's say the public address provided is 244.13.5.89. And all the machines when get to internet, they all are recognized as 244.13.5.89 despite their private address being 192.168.3.x, which is fine. But now when the router sends the request for each of these machines, how does it maps each response to respective machine?
If the answer is NAT, what entries does NAT has? Is there port no. or something else?
I will give you the idea about the complex case scenario :-
If all datagrams arriving at the NAT router from the WAN have the same
desti- nation IP address (specifically, that of the WAN-side interface
of the NAT router), then how does the router know the internal host to
which it should forward a given datagram?
The trick is to use a NAT translation table at the NAT router, and to
include port numbers as well as IP addresses in the table entries.
See more details about NAT Table here.
SORRY, don't know why it took me minutes to post an answer, it was not allowing me to add the answer...
NAT has following fields:
source computer address and website, so when it receives packet it will replace source computer address with its public address, and send it to the internet once response comes back it sees oh this come from the website and it was requested by the source computer. It will again replace destination address with source computer address and send it in LAN.
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What I want to understand is how to determine clients that associated/connected to an AP using network traffic.
For clarification: I am not connected to the network. I am using a usb card on monitor mode and capturing traffic using wireshark. I choose some AP I saw on the traffic and filtered all traffic relevant to it. I am trying to understand, using this data, what clients are connected to this AP.
Hope you guys understand what I mean.
You should look for the association request and association response 802.11 packets in your sniffed list. Basically, client sends the association request and AP replies with the response (you can parse the response according to the 802.11 to find out whether association was granted or rejected. The client will normally be connected to the AP until deauthentication packet arrives from the AP.
From what I know, there's no way to ask the AP for all connected clients unless you are the administrator of the AP but you wouldn't be listenning to the connection wouldn't you ? ;)
However, you can listen to all traffic relevant to this AP and try to determine wheter the captured packet is doing real business (fetching url, ftp, etc.) or just noise. This way, you can safely say that this given IP address is connected to the AP or not.
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Here is the scenery. I have several buildings with same subnet IP addresses (192.168.0.0/24).
I need to secure that connection with OpenVPN.
Please assist me how can I organize VPN connection with IP network 10.10.10.0/24
example
HQ (192.168.0.0/24) - VPN 1 (10.10.10.50) --- VPN 2 (10.10.10.51) - Branch (192.168.0.0/24)
I meas assist with NAT if it will be needed or if I just connect , the C class traffic will just be passed through.
Thank you
Since you are merging two nets, you should use TAP interfaces.
TAP (as in network tap) simulates a link layer device and it operates with layer 2 packets such as Ethernet frames. TUN (as in network TUNnel) simulates a network layer device and it operates with layer 3 packets such as IP packets. TAP is used to create a network bridge, while TUN is used with routing.
This way, you will be joining the HQ network with the branch network like if you plug a (secure, pre-shared key may be ok) ethernet cable between them.
But this will likely crash both environments if they are operating now without knowing one about each other. Maybe the HQ router has the same IP as the branch one, or maybe not, but they are both acting as DHCP servers and now they will be on the same network.
Do you really need to merge both nets? I think you should consider to do some network design and let the HQ on the 192.168.0.0/24 but change the n-th branch to 192.168.n.0/24. Then you can do a TUN OpenVPN setup and route between branches and HQ in a hierachical way. It will save you some bandwith too by not spreading irrelevant broadcast traffic over the tunnel.
Thank you
Well you know, I do not have any router installed in HQ side or anywhere in network and maybe that is a big minus (and I will fix that in a time). Now I just need to secure the links between HQ and branches so the optical network provider (that provides links) could not access out data. If you have any good solution please write.
Thank you
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I understand that a router uses NAT to translate the public IP we get from the ISP to say 300 local IPs. Does a switch perform the same function? If not, how's it different?
No, a switch cannot perform NAT and translate public(s) IP addresses into private addresses.
A switch is a network device that filters and forwards packets between LAN segments. Switches operate at the data link layer (layer 2). So, they are not aware of IP addresses which are network layer (layer 3). A switch keeps a record of the MAC addresses of all the devices connected to it. With this information, a switch can identify which system is sitting on which port. So when a frame is received, it knows exactly which port to send it to, without significantly increasing network response times.
Routers are network devices used to interconnect two different networks (with different IP addressing schemes).
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whats the purpose of local IP addresses if there are mac addresses? ARP maps mac addresses to IP addresses but I don't see why it's needed, because I thought data on LANs are sent as frames which only care about the mac addresses.
Long ago and far away, there was more to the world than Ethernet LANs, and application writers didn't care whether your PC was attached to an Ethernet, a Token Ring, an XNS net, or dial-up. IP provides a layer of abstraction and coherence across the top of all those and many more, allowing application authors to ignore the differences between them.
And what happens if you want to talk to a macine that isn't on your local area network (such as StackOverflow).
IP allows routing of packets anywhere, not just locally in your current network segment and, though it's mostly over Ethernet now, IP can equally well work over other underlying layers, giving a consistent view to the upper layers. This is vital given how much stuff is actually built on IP (DNS, FTP, SSH, HTTP and so on).
Machines almost certainly will cache IP-to-ethernet details to speed up subsequent transfers so the impact of translation on the LAN isn't so bad.