Gratuitous ARP

[169.254.X.Y]

Lastly, this is probably really a stupid question. Recently, I figured out that ARP has four brothers. The one I discovered by playing with GNS3 was 'Gratuitous ARP'. I am not sure but this is how I understood the gratuitous arp. 

If I try to use a public IP which someone is already using, I won't be able to use it because of gratuitous ARP. I heard that 1 public ip costs around 50 bucks (not sure). I guess that ISPs have a huge public ip pools and obviously some of public ip addresses are not being used. If I can figure out that whether a certain public ip is being used or not through gratuitous ARP, is it possible to use a public ip without paying 50 bucks to ISP?   

[Flavio Miranda]

"He said BPDU is OSI 7 layer " for that one he is wrong. Bridge Protocol Data Unit is not Layer 7 but Layer 2, dont matter which concept you are using.

 About which one to use, sincerelly, I dont think it is relevant. If you understand the concept you can refers to any standard.

 OSI model is more detailed but at the end, both do the same thing. 

About your question related to IP address, it is not possible. A simple Access List on the ISP Route is enough to limit you to the IP range you´ve been paying. Unless your ISP is too stupid.

 There´s no free lunch my friend, specially on technology world!

[169.254.X.Y]

He actually said that BPDU is from OSI model and other protocols I have learned from ICDN 1 are from TCP/IP.

[169.254.X.Y]

LOL :) Thank you very much.

[Peter Paluch]

Hi,

I agree with Flavio. Let me also share my point of view.

The issue of whether we are using TCP/IP or OSI depends on what protocol stack we are looking at. If we take the TCP/IP family of protocols, then by all means, they are just that - TCP/IP. They are not OSI protocols; TCP/IP creators did not attempt to design their stack by the OSI rules. However, because the OSI model talks about natural and obvious processes that must take place when two nodes communicate across a network, you can draw analogies and parallels between TCP/IP and OSI, and try to match layers between them - but they are not similar by intention; they are similar simply because they have to accomplish the same goal, otherwise we would have no network communication at all.

OSI, however, is much more than just an educational model - it is actually a formal specification of an entire networking stack, and there are real protocols designed strictly according to the OSI model. They have been used mostly in telecommunication networks and aviation (the ATN), though some have been adopted and adapted for use in TCP/IP world, too (most notably, the IS-IS routing protocol).

The reason why TCP/IP and OSI exist is simply because they have been developed independently, in parallel, and around the same time - TCP/IP grew up from the DARPA project while OSI was an industry effort. There was a notion that once OSI is fully developed and standardized, it will replace TCP/IP, but by the time it was truly ready, TCP/IP was so popular it could not be thrown out of window anymore. Environments that felt they need to follow a very strict and rigidly standardized ruleset would opt for OSI, that's why telecommunications went with OSI. Otherwise, TCP/IP was much easier to implement and get running, as it was no-nonsense, no-fuss, straight-to-point type of networking stack.

As for the BPDU question - there is absolutely no way a BPDU (or STP for that matter) could be on OSI Layer7. Just as Flavio mentioned, it is Layer2, period. STP is a protocol that creates a loop-free Layer2 topology, and so is a Layer2 control protocol.

The "Gratuitous ARP" has nothing to do with using public IPs. Check out this Wikipedia section for an introduction into Gratuitous ARP:

https://en.wikipedia.org/wiki/Address_Resolution_Protocol#ARP_announcements

Feel welcome to ask further!

Best regards,
Peter

[169.254.X.Y]

Thank you very much. She actually said that BPDU is from OSI model and other protocols I have learned from ICDN 1 are from TCP/IP. I honestly don't get, but she concluded that protocols from OSI model are used to create a net and protocls from TCP/IP are used for communication.