I have studied from assorted sources that router is a great 3 device. That does routing based on Network class (layer 3) header, but I have a doubt.

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Network address translation (NAT) is a function of Router i beg your pardon is compelled for routing traffic. That does processing based upon both Network layer and also Transport class (layer 4) headers. So why cant we say that router is a great 3 & great 4 maker when the is handling layer 4 header also?



Network attend to translation (NAT) is a attribute of Router i m sorry isrequired because that routing traffic.

That is totally incorrect. NAT is a kludge (a clumsy, inefficient solution) design to expand the life the IPv4 addressing till IPv6 is ubiquitous. NAT breaks the IP paradigm of end-to-end connectivity, and many points have troubles with NAT. Routing works quite well there is no NAT. You must only use NAT were you absolutely should use it, e.g. Exclusive to public addressing, or because that joining two networks with overlapping addresses.

It does processing based upon 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 usage the TCP or UDP ports, or ICMP questions IDs, however it only works because that those three protocols, and also it breaks various other transport protocols and also many applications. A routing table (what routers usage to recognize the route of packets) has actually nothing about layer-4 protocols in it.

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

4.1.2. Network address Port translate into (NAPT)

NAPT extend the notion of translate in one step further by alsotranslating move identifier (e.g., TCP and UDP port numbers,ICMP query identifiers). This allows the deliver identifiers that anumber of exclusive hosts to be multiplexed into the transportidentifiers that a single external address. NAPT allows a collection of hoststo re-publishing a single external address. Keep in mind that NAPT can be combinedwith simple NAT so that a pool of external addresses are offered inconjunction through port translation.

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For packets outbound from the exclusive network, NAPT would translatethe resource IP address, resource transport identifier and also related fieldssuch together IP, TCP, UDP and also ICMP header checksums. Transport identifiercan be one of TCP/UDP harbor or ICMP questions ID. For inbound packets, thedestination IP address, location transport identifier and also the IPand carry header checksums are translated.