31 Days Before Your CCNA Exam

300 31 Days Before Your CCNA Exam1. PC1 and PC2 send packets destined for the Internet.2. When the packets arrive at R2, NAT overload changes the source address to the inside globalIP address and keeps the assigned port numbers (1555 and 1331 in this example) to identifythe client from which the packet originated.3. R2 updates its NAT table. Notice the assigned ports. R2 then routes the packets to theInternet.4. When the web server replies, R2 uses the destination source port to translate the packet tothe correct client.NAT overload attempts to preserve the original source port. However, if this source port is alreadyused, NAT overload assigns the first available port number starting from the beginning of theappropriate port group 0 to 511, 512 to 1023, or 1024 to 65535.NAT BenefitsThe benefits of using NAT include the following:■■■■NAT conserves registered IP address space because, with NAT overload, internal hosts canshare a single public IP address for all external communications.NAT increases the flexibility of connections to the public network. Multiple pools, backuppools, and load-balancing pools can be implemented to ensure reliable public network connections.NAT allows the existing scheme to remain while supporting a new public addressing scheme.This means an organization could change ISPs and not need to change any of its insideclients.NAT provides a layer of network security because private networks do not advertise theirinside local addresses outside the organization.NAT LimitationsThe limitations of using NAT include the following:■■■■■Performance is degraded: NAT increases switching delays because translating each IPaddress within the packet headers takes time.End-to-end functionality is degraded: Many Internet protocols and applications depend onend-to-end functionality, with unmodified packets forwarded from the source to the destination.End-to-end IP traceability is lost: It becomes much more difficult to trace packets thatundergo numerous packet address changes over multiple NAT hops, making troubleshootingchallenging.Tunneling is more complicated: Using NAT also complicates tunneling protocols, such asIPsec, because NAT modifies values in the headers that interfere with the integrity checksdone by IPsec and other tunneling protocols.Services may be disrupted: Services that require the initiation of TCP connections from theoutside network, or stateless protocols such as those using UDP, can be disrupted.