Netopia R7200 manual Understanding Netopia NAT Behavior C-3

Page 223

Understanding Netopia NAT Behavior C-3

When the Netopia R7200 receives this IP packet, it cannot simply forward it to the WAN interface and the Internet since the IP addresses on the LAN interface are not valid or globally unique for the Internet. Instead, the Netopia R7200 has to change the IP packet to reflect the IP address that was acquired on the WAN interface from the ISP.

The Netopia R7200 will first substitute the source IP address with the IP address that was acquired on the WAN interface, which in this case is 200.1.1.40. Next the Netopia R7200 will substitute the source TCP or UDP port with a TCP or UDP port from within a specified range maintained within the Netopia R7200. And finally the modified IP packet's checksum is recalculated (as specified in RFC 1631) and the packet is transmitted across the WAN interface to its destination, the WWW server on the Internet.

If the send and response IP packets were drawn out, this process would look like the following:

WWW Server

ISP Router

Netopia Router

 

Workstation A

163.176.4.32

200.1.1.1

 

LAN: 192.168.5.1

192.168.5.2

 

 

 

 

 

WAN: 200.1.1.40

 

 

 

 

 

 

 

Router

 

 

Netopia

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ISP Router to WWW

Netopia to ISP Router

 

 

Wkstn A to Netopia

Src IP: 200.1.1.40

Src IP: 200.1.1.40

 

 

Src IP: 192.168.5.2

Dst IP: 163.176.4.32

Dst IP: 163.176.4.32

 

 

Dst IP: 163.176.4.32

Src Port: 5001

Src Port: 5001

 

 

Src Port: 400

Dst Port: 80

Dst Port: 80

 

 

Dst Port: 80

WWWto ISP Router Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port: 80

Dst Port: 5001

ISP Router to Netopia Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port: 80

Dst Port: 5001

Netopia to Wkstn A Src IP: 163.176.4.32 Dst IP: 192.168.5.2 Src Port: 80

Dst Port: 400

As you can see, the IP packet from Workstation A is sent to the Netopia R7200 and the source IP address is substituted with 200.1.1.40 and the source port is substituted with 5001, then the IP packet checksum is recalculated. When this modified packet reaches the WWW server on the Internet, the WWW server responds and sends the IP packet back to destination IP address 200.1.1.40 and destination port 5001.

When the Netopia R7200 receives this IP packet from the WWW server, the Netopia R7200 replaces the destination IP address with 192.168.5.2, the address for Workstation A. The port is changed back to 400, the IP packet checksum is recalculated, and the IP packet is sent to Workstation A on the Netopia R7200s LAN interface.

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Contents Netopia R7200 Sdsl Router Part Number Contents Part II Advanced Configuration Contents Aurp Snmp Xmodem Part III Appendixes User’s Reference Guide Part I Getting Started User’s Reference Guide Chapter Introduction Features and capabilitiesOverview How to use this guide Finding an Internet service provider Chapter Setting Up Internet ServicesEndorsements Deciding on an ISP account Setting up a Netopia R7200 account Obtaining an IP addressUnique requirements Pricing and supportWithout Network Address Translation Obtaining information from the ISPLocal LAN IP address information to obtain With Network Address TranslationUser’s Reference Guide Find a location Chapter Making the Physical ConnectionsWhat you need Identify the connectors and attach the cablesYour router on Netopia R7200 Sdsl Router back panel portsNetopia R7200 LED front panel Netopia R7200 Sdsl Router status lightsReadying computers on your local network Chapter Connecting to Your Local Area NetworkUser’s Reference Guide Connecting to an Ethernet network 10Base-T Adding an external modem Connecting to a LocalTalk network Chapter Setting up your Router with the SmartStart Wizard Before running SmartStart SmartStart Wizard configuration screens Setting up your Router with the SmartStart WizardEasy option Advanced setupAdvanced option Configuration screen on Configuration tab Dynamic configuration recommendedStatic configuration optional Add. Repeat this process for the secondary DNS TCP/IP Configuring TCP/IP on Macintosh computersTCP/IP or MacTCP Dynamic configuration using MacIP optional Setting up your Router with the SmartStart Wizard User’s Reference Guide Chapter Console-Based Management Connecting through a Telnet session Configuring Telnet software Connecting a console cable to your routerPC ANSI-BBS Navigating through the console screensEasy Setup console screens Chapter Easy SetupAccessing the Easy Setup console screens See Appendix A, Troubleshooting, for more suggestions Quick Easy Setup connection path Main Menu appears Sdsl Line ConfigurationPrevious Screen Next Screen Easy Setup ProfileIP Easy Setup Easy Setup Security Configuration Previous Screen To Main Menu Part II Advanced Configuration User’s Reference Guide WAN configuration Chapter WAN and System ConfigurationATM VPI ATM VCI Creating a new Connection Profile IPX Profile Parameters Remote IPX Network Datalink PPP/MP Options Data Compression Default profile Main Menu Default Profile screen appears IP parameters default profile screen System configuration screens IPX parameters default profile screenNavigating through the system configuration screens System configuration features Date and time Network protocols setupFilter sets firewalls IP address servingConsole configuration Logging Upgrade feature setSnmp Simple Network Management Protocol SecurityInstalling the Syslog client User’s Reference Guide Chapter Line Backup Backup Configuration screen appears Atdt Backup Configuration screenIP Setup screen Connection Profiles Using Scheduled Connections with BackupScheduled Connections screen appears Management/Statistics Force Recovery Event Logs QuickViewSnmp Support User’s Reference Guide Network Address Translation features Chapter IP Setup and Network Address TranslationHOW NAT Works Previous Screen Using Network Address TranslationV2 multicast Numbered Sdsl WAN1 Associating port numbers with nodesAdvanced IP/IPX router configuration options Data Link Options IP Setup and Network Address Translation Network Address Translation guidelines IP setup User’s Reference Guide Select Add Export. The Add Exported Service screen appears Select Service. a pop-up menu of services and ports appears IP subnets User’s Reference Guide Static routes Viewing static routes Static Routes screen will appearAdding a static route Modifying a static route Rules of static route installationDeleting a static route Main Menu System Configuration IP Address Serving 176.163.222.10 Dhcp NetBios Options Serve Bootp Clients IP Address Pools IP Setup and Network Address Translation Dhcp NetBIOS Options NetBios Type MacIP KIP forwarding setup You have finished your IP setup User’s Reference Guide Internetwork Packet Exchange IPX Chapter IPX SetupIPX features IPX definitionsRouting Information Protocol RIP Service Advertising Protocol SAPIPX address SocketIPX setup screen NetBIOSIPX spoofing Default Gateway Address IPX routing tables User’s Reference Guide AppleTalk networks Chapter AppleTalk SetupAppleTalk protocol AT Routing Table Routers and seeding MacIPInstalling AppleTalk Upgrade Feature Set Configuring AppleTalk EtherTalk setupLocalTalk setup Viewing Aurp partners Aurp setupAurp Free Trade Zone Modifying an Aurp partner Adding an Aurp partnerReceiving Aurp connections Deleting an Aurp partnerConfiguring Aurp Options Aurp Options Tickle Interval Hhmmss User’s Reference Guide Quick View status overview Chapter Monitoring ToolsGeneral status Status lights Current statusGeneral Statistics Statistics & LogsPhysical Interface Event historiesNetwork Interface WAN Event History Device Event History Routing tablesIP routing table IPX Sap Bindery table IPX routing tableAppleTalk routing table Served IP Addresses IP Address Lease Management screen appears Snmp System InformationCommunity strings Snmp Setup screenSnmp traps Deleting IP trap receivers Setting the IP trap receiversViewing IP trap receivers Modifying IP trap receiversWeb-based management pages Event History pages WAN Event History Device Event History User’s Reference Guide Suggested security measures Chapter SecurityUser accounts Protecting the configuration screens Protecting the Security Options screenDial-in console access What’s a filter and what’s a filter set? Enable SmartStart/SmartView/Web serverTelnet access About filters and filter setsFilter priority How filter sets workHow individual filters work Parts of a filter filtering rulePort numbers Other filter attributes Port number comparisonsUDP Putting the parts togetherFiltering example #2 Filtering example #1Disadvantages of filters Design guidelinesWorking with IP filters and filter sets An approach to using filtersNaming a new filter set Adding a filter setAdding filters to a filter set Input and output filters-source and destinationADD this Filter NOW Cancel Deleting filters Viewing filter setsViewing filters Modifying filtersDeleting a filter set Modifying filter setsSample IP filter set TCP Icmp UDP Possible modifications User’s Reference Guide IPX filters Viewing and modifying packet filters IPX packet filtersAdding a packet filter Adding a packet filter set IPX packet filter setsDeleting a packet filter Viewing and modifying packet filter setsNo Match Deleting a packet filter set IPX SAP filtersViewing and modifying SAP filters Deleting a SAP filter Adding a SAP filterViewing and modifying SAP filter sets IPX SAP filter setsAdding a SAP filter set Deleting a SAP filter set Basic IP packet components Firewall tutorial General firewall termsBasic protocol types Example TCP/UDP Ports Firewall design rulesFirewall Logic Logical and function Binary representationEstablished connections Implied rulesExample IP filter set screen Example network Filter basicsExample Example filtersExample Example Chapter Utilities and Diagnostics Ping Receive return Ping packet Stop Ping Trace RouteTelnet client Disconnect Telnet console session Factory defaultsUpdating firmware Transferring configuration and firmware files with TftpDownloading configuration files Transferring configuration and firmware files with Xmodem Uploading configuration filesIdle Do you want to send a saved configuration to your Netopia? Restarting the system Part III Appendixes User’s Reference Guide Configuration problems Appendix a TroubleshootingNetwork problems Console connection problemsPower outages How to reset the router to factory defaultsEnvironment profile How to reach usTechnical support Before contacting NetopiaFAX-Back Online product informationUser’s Reference Guide Appendix B Understanding IP Addressing What is IP?About IP addressing Subnets and subnet masks Subnet masks Example Using subnets on a Class C IP internetISP Network Network configurationDistributing IP addresses Example Working with a Class C subnetBackground Technical note on subnet masking Configuration Netopia R7200 Dhcp server characteristicsDhcp address serving Serve dynamic WAN clients Using address servingManually distributing IP addresses MacIP servingUnderstanding IP Addressing B-9 Tips and rules for distributing IP addressesDhcp example Internet Nested IP subnets0.0 C.1 WAN 3719 Packet header types BroadcastsUser’s Reference Guide Network configuration Appendix C Understanding Netopia NAT BehaviorBackground User’s Reference Guide Understanding Netopia NAT Behavior C-3 Netopia Router WWW Server ISP Router LAN Understanding Netopia NAT Behavior C-5 Exported servicesImportant notes Understanding Netopia NAT Behavior C-7 Summary Appendix D Binary Conversion Table Decimal Binary Appendix E Further Reading User’s Reference Guide Further Reading E-3 User’s Reference Guide Pinouts for Auxiliary port modem cable Appendix F Technical Specifications and Safety InformationDescription Power requirementsEnvironment Software and protocols International Agency approvalsRegulatory notices North AmericaDeclaration for Canadian users Important safety instructionsBattery Telecommunication installation cautionsAppendix G About Sdsl User’s Reference Guide Glossary User’s Reference Guide Glossary User’s Reference Guide Remapping See network number remapping User’s Reference Guide Glossary User’s Reference Guide Numerics IndexIndex-2 Index-3 Index-4 Index-5 Index-6 Limited Warranty and Limitation of Remedies User’s Reference Guide
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