BROCADE IP PRIMER

Console49F Link 50F1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47Power1F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48BI4XGBI4XGBI24CBI24CBI24C16T X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RX12BigIron RX-8NETWORKS7131819242530313637424348AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMBI24CBI24CBI24CBI24CBI24CBI4XGBI4XGT X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RXEJECT SYSPacket Capturingreceive any traffic bound for the public PAT address using TCP destination port80, forward it to the Web server. If you receive any traffic bound for the publicPAT address using TCP destination port 25, forward it to the mail server. If youreceive any traffic bound for the public PAT address using UDP destination port53, forward it to the DNS server.” And that's essentially what you do with PAT.DestinationTCP 80192.168.100.5DestinationTCP 801.1.2.10192.168.100.1/24 1.1.2.1/24INTERNET192.168.100.5/24DestinationTCP 25192.168.100.10DestinationUDP 53192.168.100.15DestinationTCP 251.1.2.10DestinationTCP 531.1.2.10192.168.100.10/24192.168.100.15/24What if you wanted to add a second Web server into the mix? Well, you're outof luck. You can only forward one incoming Layer 4 port to one destination.One way that you could get around this is to use a different port (say TCP 81),and have it translate back to the second web server on TCP destination port80. The problem with this method is that you'd have to train your users toaccess the website by specifying port 81 when they put it in their browsers. Inthe end, you're best served by using NAT in that scenario (giving each webserver its own public NAT address).To summarize, the whole point of NAT and PAT is to be able to translate IPaddresses to appear to come from another network. NAT allows you to accomplishthis at Layer 3, using a one-to-one relationship. PAT allows you toaccomplish this at Layer 4, using a many-to-one relationship.Packet CapturingWhether you're trying to troubleshoot a network problem, or just trying tounderstand your network's normal traffic better, packet capturing is yourfriend. The idea behind packet capturing is to let you see the packets as theytraverse the network. This is usually done with packet capturing software.The term “packet capturing” is a bit of a misnomer. All of the packet capturingsoftware packages that I know actually capture frames (Layer 2). This, ofcourse, includes the frame header, the packet header, the segment header,and the original data generated by Layer 7. Even though “packet capture” isthe popular term, you're actually capturing frames.Picture three servers plugged into an Ethernet hub (not a switch, a hub). You'rethe network administrator, and you want to view how the three servers arecommunicating among themselves. You plug your laptop into the hub. Youknow that a hub receives a frame, and then sends a copy of the frame outevery one of its ports. You also know that, by default, your laptop's NIC will onlyBrocade IP Primer 65