CompTIA Network+ All-in-One Exam Guide in Software

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CompTIA Network+ All-in-One Exam Guide
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Network ID: 19216810/24
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Default gateway 192168101
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Redone network IDs; nodes in the LAN use private IP addresses internally
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Packet Info Source IP: 123138 Ephemeral port: 1176 Destination IP: 124365223 Destination port: 80
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Router IP: 192168101 Incoming packet from 192168102 Recording ephemeral source port 1176 meant for 124365223:80 Replacing source IP with my IP address NAT Translation Table Ephemeral source port: 2001 Destination IP: 124365223
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Packet Info Source IP: 123138 Ephemeral port: 1176 Destination IP: 124365223 Destination port: 80
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Figure 8-16
NATing a packet
8: The Wonderful World of Routing
When the receiving system sends the packet back, it reverses the IP addresses and ports The overloaded NAT router compares the incoming destination port and source IP address to the entry in the NAT translation table to determine which IP address to put back on the packet (Figure 8-17) It then sends the packet to the correct computer on the network
Router IP: 192168101 Incoming packet from 124365223 According to NAT translation table, this should go to 192168102 Replacing source IP with my IP address NAT Translation Table Ephemeral source port: 2001 Destination IP: 124365223
Packet Info Source IP: 192168101 Source port: 80 Destination IP: 192168102 Destination port: 1176
Packet Info Source IP: 124365223 Source port: 80 Destination IP: 1241 Destination port: 1176
Figure 8-17 Updating the packet
Overloaded NAT takes care of all of the problems facing a network exposed to the Internet You don t have to use legitimate Internet IP addresses on the LAN and the IP addresses of the computers behind the routers are invisible and protected from the outside world Since the router is revising the packets and recording the IP address and port information already, why not enable it to handle ports more aggressively Enter port forwarding, stage left Port Forwarding Port forwarding hides a port number from the wilds of the Internet, enabling public servers to work behind a NAT router Port forwarding gives servers the protection of NAT while still allowing access to that server Suppose you have a Web server behind a NAT router You know from earlier in the book that Web servers look for incoming port 80 addresses A port-forwarding router recognizes all incoming requests for a particular port and then forwards those requests to an internal IP address To support an internal Web server, the router is configured to forward all port 8080 packets to the internal Web server at port 80, as shown in Figure 8-18 Port Address Translation Different manufacturers use the term Port Address Translation (PAT) to refer to both overloaded NAT and port forwarding, though not at the same time The Cisco router in Figure 8-19, for example, calls their overloaded NAT Port Address Translation (PAT)
CompTIA Network+ All-in-One Exam Guide
Figure 8-18
Setting up port forwarding on a home router
Figure 8-19 Configuring Port Address Translation on a Cisco router
8: The Wonderful World of Routing
EXAM TIP The CompTIA Network+ exam follows the Cisco definition of Port Address Translation, making the term synonymous with overloaded NAT
Configuring NAT
Configuring NAT on home routers is a no-brainer as these boxes invariably have NAT turned on automatically Figure 8-20 shows the screen on my home router for NAT Note the radio buttons that say Gateway and Router
Figure 8-20 NAT setup on home router
By default the router is set to Gateway, which is Linksys-speak for NAT is turned on If I wanted to turn off NAT, I would set the radio button to Router Commercial-grade routers use NAT more explicitly, enabling you to do Static NAT, Pooled NAT, port forwarding, and more Figure 8-21 shows a router configuration screen on a Cisco router
Figure 8-21 Configuring NAT on a commercial-grade router
CompTIA Network+ All-in-One Exam Guide
Dynamic Routing
Based on what you ve read up to this point, it would seem that routes in your routing tables come from two sources: either they are manually entered or they are detected at setup by the router In either case, a route seems to be a static beast, just sitting there and never changing And based on what you ve seen so far, that is absolutely true Routers have static routes But most routers also have the capability to update their routes dynamically, assuming they re provided with the extra smarts in the form of dynamic routing protocols If you ve been reading carefully you might be tempted at this point to say: Why do I need this dynamic routing stuff Don t routers use metrics so I can add two or more routes to another network ID in case I lose one of my routes Yes, but metrics really only help when you have direct connections to other network IDs What if your routers look like Figure 8-22
Figure 8-22
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