I have actually studied from various sources the router is a class 3 device. It does routing based upon Network layer (layer 3) header, however I have actually a doubt.

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Network deal with translation (NAT) is a attribute of Router i m sorry is required for routing traffic. That does processing based upon both Network layer and also Transport great (layer 4) headers. Therefore why cant us say the router is a great 3 & great 4 machine when the is handling layer 4 header also?



Network resolve translation (NAT) is a feature of Router which isrequired for routing traffic.

That is totally incorrect. NAT is a kludge (a clumsy, inefficient solution) draft to prolong the life the IPv4 addressing until IPv6 is ubiquitous. NAT breaks the IP paradigm that end-to-end connectivity, and also many points have problems with NAT. Routing works rather well there is no NAT. You have to only usage NAT were you absolutely have to use it, e.g. Private to windy addressing, or because that joining 2 networks through overlapping addresses.

It walk processing based on both Network layer and also Transport layer(layer 4) headers.

Again, no. Routing does not involve the layer-4 header. One type of NAT, NAPT (Network address Port Translation), will use the TCP or UDP ports, or ICMP questions IDs, yet it only works because that those 3 protocols, and it breaks various other transport protocols and also many applications. A routing table (what routers use to identify the course of packets) has actually nothing about layer-4 protocols in it.

RFC 2663, IP Network resolve Translator (NAT) Terminology and Considerations describes NAPT:

4.1.2. Network deal with Port translate in (NAPT)

NAPT extends the id of translate in one step further by alsotranslating move identifier (e.g., TCP and also UDP port numbers,ICMP ask identifiers). This permits the carry identifiers that anumber of exclusive hosts to it is in multiplexed into the transportidentifiers that a single external address. NAPT permits a collection of hoststo re-publishing a solitary external address. Note that NAPT deserve to be combinedwith straightforward NAT so that a swimming pool of exterior addresses are offered inconjunction v port translation.

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For packets outbound from the exclusive network, NAPT would certainly translatethe resource IP address, source transport identifier and related fieldssuch as IP, TCP, UDP and ICMP header checksums. Transport identifiercan be one of TCP/UDP port or ICMP questions ID. For inbound packets, thedestination IP address, destination transport identifier and the IPand deliver header checksums space translated.