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Table 6-2 Mapping of Original to Externally Addressable IP Addresses
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Original IP Address Address Used for External Communication
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192168151 192168152 192168153
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7494053:58250 7494053:58251 7494053:58252
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7494053:58250 192168151 7494053:58251 749405358251 7494053:58252
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7494053:58251
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FIGURE 6-12 A Single Address NAT in progress The NAT device maintains a table of ports
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for its single external address of 7494053 that maps to the computers on the internal network
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IP facilitates communication, but it s a best-effort service with no guarantee of delivery It s like tying a letter you want to deliver around your cat s neck, telling him the address, and then setting the cat about his way (You re assuming it s an intelligent cat) You have no way of knowing if the letter got there That s IP the packet goes out onto the network, but you get no veri cation of delivery IP packets can be corrupted, lost, arrive at the destination out of order, or get delivered multiple times The transmission of an IP packet and its routing is enabled via its header, which is simple (see Figure 6-13)
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Transmission Control Protocol (TCP)
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Version (4)
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Type Of Service (TOS)
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Size of Datagram (header and the data)
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Identification
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Flags
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Time To Live (TTL)
Protocol (1=ICMP, 6=TCP)
Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any, the IP Header Length [IHL] is greater than 5)
Data
FIGURE 6-13 IP header format As you can see, not much is in here You can tell it the type of protocol, the size of the packet, where it is from, and where it s going The Time to Live (TTL) says how many hops between routers the packet can survive Each hop at a router decreases the TTL by 1, and when it reaches 0, it stops being routed This is designed to stop packets from being routed in a circle inde nitely Although IP allows communication and actual data transfer, other protocols that sit above it in the Transport layer of the OSI model are needed
Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP) sits on top of IP in the OSI stack and adds reliability to IP s communication capabilities TCP creates a connection between the hosts communicating and provides guaranteed data delivery by resending data packets if they don t arrive TCP also ensures the data is presented in the correct order and follows a sequence of creating the connection, having a data transfer session, and then terminating the connection after the transfer is complete Each of these connections uses a TCP port, which can be between 1 and 65535 Some of these ports are reserved for speci c applications types that listen on a port for requests For example:
6 TCP/IP
Port 21 File Transfer Protocol (FTP) Port 23 Telnet for character-based communication to a remote computer
6
TCP/IP
Port 25 Simple Mail Transfer Protocol (SMTP) Port 80 Hypertext Transfer Protocol (HTTP) the most used because it powers all those Web browser sessions When you tell your Web browser to go to http://wwwmsncom, you are telling it to use protocol http to talk to server wwwmsncom The Web browser opens a request to port 80 on server wwwmsncom
TCP is great because it adds reliability to your services, but it adds overhead in terms of data size sent over the network, and the time it takes to initiate the connection before useful data transfer can start Figure 6-14 shows the TCP header Notice it does not have details about the source or destination address This is because the TCP header just sits inside the IP data, which already contains the source and destination information Notice you have port information now along with sequence space
0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1