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

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Network attend to translation (NAT) is a feature of Router i beg your pardon is compelled for routing traffic. The does processing based upon both Network layer and also Transport class (layer 4) headers. For this reason why cant we say the router is a layer 3 & class 4 an equipment when the is handling layer 4 header also?



Network address translation (NAT) is a function of Router i beg your pardon isrequired for routing traffic.

That is fully incorrect. NAT is a kludge (a clumsy, inefficient solution) draft to prolong the life that 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 fairly well there is no NAT. You must only usage NAT to be you absolutely must use it, e.g. Personal to windy addressing, or because that joining two networks v overlapping addresses.

It go processing based upon both Network layer and Transport layer(layer 4) headers.

Again, no. Routing does no involve the layer-4 header. One form of NAT, NAPT (Network attend to Port Translation), will usage the TCP or UDP ports, or ICMP questions IDs, however it only works for those 3 protocols, and it breaks various other transport protocols and many applications. A routing table (what routers use to identify the route of packets) has actually nothing about layer-4 protocols in it.

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

4.1.2. Network address Port translation (NAPT)

NAPT extend the concept of translation one step more by alsotranslating carry identifier (e.g., TCP and also UDP harbor numbers,ICMP query identifiers). This enables the transfer identifiers the anumber of private hosts to be multiplexed right into the transportidentifiers the a single external address. NAPT permits a collection of hoststo re-publishing a single external address. Keep in mind that NAPT can be combinedwith an easy NAT so that a pool of exterior addresses are offered inconjunction v port translation.

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For packets outbound indigenous the personal network, NAPT would certainly translatethe source IP address, source transport identifier and related fieldssuch as IP, TCP, UDP and ICMP header checksums. Carry identifiercan be one of TCP/UDP harbor or ICMP ask ID. For inbound packets, thedestination IP address, location transport identifier and also the IPand move header checksums are translated.