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We know that Router can't be used for converting different model (e.g OSI model to TCP/IP or TCP/IP model to OSI) of network. It isn't a multiprotocol converter. But gateway is a highly sophisticated router which can be used for connecting different model of network. I have been finding actual reason behind this but failed to find any fruitful stuff. Can anybody help to make out above concepts.

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    Router and Gateway are defined based on their capabilities and how they work. If a router would be defined differently it could do other things. So you are basically asking why specific capabilities are called a specific name. Note also that meaning can change over time, i.e. what is today called a router is way more than what was traditionally called a router. Sep 10 at 20:35
  • I think you might be misunderstanding a few things. OSI model is used today mostly as a reference, TCP/IP is the name of the game. Any router that is forwarding traffic between a subnet and the rest of the world is a "gateway". You can configure a RasPi to forward traffic into and out of a subnet with 2 static routes, in which case it's a gateway for that subnet. An example of OSI protocol is IS-IS, which for example Cisco routers support. They can also FW between between RS-232 and TCP/IP. So please expand your question, add some examples from real life to clarify what you mean. Sep 10 at 20:47
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Models are models - concepts for thought. Like philosophies - sometimes one fits better, some other time the other one.

Converting between TCP/IP model and OSI model doesn't make sense. It's like trying to convert a glass that's half full to a glass that's half empty. It's only a matter of perspective.

The OSI and TCP/IP models are actually very similar. OSI is more detailed (and more theoretical), and TCP/IP focuses on the network (OSI)/internet (IP) and the transport layer, naturally. OSI splits IP's application layer into application, presentation and session (not too common in real life), and IP's link layer is represented by OSI's data link and physical layers (very useful in practice).

On the network layer, router and gateway are the very same thing. There are various feature levels (concerning filtering, firewalling, inspection, address translation, ...) but both terms are interchangeable still.

Gateway can also refer to various other concepts that connect different realms. Gateways from different OSI layers (bridges, routers, proxies, application-layer gateways, ...) are generally different, but even gateways working in the same layer can be vastly different.

Higher-layer gateways can even be used to translate between protocols with a similar purpose, like an FTP-over-HTTP proxy. But all that is off-topic here for working above the transport layer.

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    please correct the spelling of "glas"
    – Alok Maity
    Sep 11 at 5:13
  • please elaborate little bit "Higher-layer gateways can even be used to translate between similar protocols, like an FTP-over-HTTP proxy". Here FTP and htttp similar protocol?
    – Alok Maity
    Sep 17 at 20:46
  • No, FTP and HTTP are completely different. However, some HTTP proxies support GET ftp://server/path requests.
    – Zac67
    Sep 17 at 20:54
  • why you written "similar protocol"? Actually I don't understand this line..
    – Alok Maity
    Sep 17 at 20:58
  • They're similar in that they both can be used to transfer files. But they work completely differently.
    – Zac67
    Sep 17 at 21:03
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I would dare say that you are the victim of the simplistic view presented by network 101 (introduction to networking) courses.

Let me start by quoting from ISO 7498 (the OSI reference model).

The purpose of this Reference Model of Open Systems Interconnection is to provide a common basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall Reference Model.

Therefore, it's not a technology, nor is it an architecture, but a reference against which to build and compare architectures.

I would continue by pointing out that gateway and router have been used interchangeably. RFC 791, the original RFC behind IP, defines the gateway as follows:

Gateways implement internet protocol to forward datagrams between networks. Gateways also implement the Gateway to Gateway Protocol (GGP) [7] to coordinate routing and other internet control information.

Today, gateway is too overloaded to have any real meaning, without a full context being given. On the other hand, the term router has a sharper profile. Primarily, it is an IP packet switch, that switches a packet from an ingress port to an egress port according to the rules it has gathered into the forwarding information base (FIB). Traditionally, and still widely valid, the source of the rules is the data exchanged through the use of a routing protocol and processed by a routing algorithm. The result of the processing is stored in a routing information base (RIB). The router selects a specific entry from the RIB, according to configured criteria, for inclusion in the FIB.

To give one example of the use of gateway, there is an application-level gateway (ALG) used in network address and port translation (NAPT). The ALG inspects the protocol data unit (PDU) relevant to the application (e.g. active FTP) and converts private address information embedded in the PDU into public address information, to enable the function of FTP applications across a NAPT device.

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  • Erm... An ALG doesn't really have any relation to NAPT. A NAPT gateway translates between different address domains (usually private and public IP) on the network and transport layers. An ALG inspects and filters (and possibly manipulates) traffic on the application layer. Both can be used entirely independently of each other.
    – Zac67
    Sep 10 at 20:57
  • Not sure about your comment. Are you sure we aren't getting caught up in another round of terminological overload? I've seen ALG used for this specific example. Sep 10 at 21:00
  • In that scenario, an ALG is likely more a proxy than a NAPT gateway.
    – Zac67
    Sep 10 at 21:01
  • I wouldn't say so. The function I've described is transparent; proxies I'm aware of (at this very moment, at least) are visible in the end-to-end path. Sep 10 at 21:03
  • Proxies can also be transparent. Pretty much any decent firewall has that option.
    – Zac67
    Sep 10 at 21:07

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