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  1. Introduction
  2. Network Topology
  3. Hardware Connections
  4. TCP/IP Ports and Addresses
  5. Network Protocol Levels
  6. Data Link Layer and IEEE
  7. Network Protocol Categories
  8. Repeaters, Bridges, Routers
  9. ARP and RARP Address Translation
  10. Basic Addressing
  11. IP (Network)
  12. TCP (Transport)
  13. UDP (Transport)
  14. ICMP
  15. Hardware Cabling
  16. Wireless media
  17. Outside Connections
  18. Ethernet
  19. Token Ring
  20. ARCnet
  21. AppleTalk
  22. FDDI
  23. IPX/SPX
  24. NetBEUI
  25. AppleTalk
  26. SNA
  27. Others
  28. Simple Routing
  29. More Complex Routing
  30. IP Masquerading
  31. Firewalls
  32. Domain Name Service (DNS)
  33. Virtual Private Networking
  34. DHCP
  35. BOOTP
  36. RPC and NFS
  37. Broadcasting and Multicasting
  38. IGMP
  39. Dynamic Routing Protocols
  40. Simple Mail Transfer Protocol (SMTP)
  41. Simple Network Management Protocol
  42. Network Services
  43. Installing Drivers
  44. Network Operating Systems
  45. Applications
  46. Wide Area Networks
  47. Backing up the network
  48. Fault Tolerance
  49. Troubleshooting
  50. Commonly used Network Ports
  51. Networking Terms and Definitions
  52. Networking RFCs and Protocols
  53. Further Reading
  54. Credits

Network Services

Networking Services and Ports

There are two general types of network services, which are connection less and connection oriented. Connection oriented service performs connection establishment, data transfer, and connection termination.


The "ping" program uses ICMP echo message requests and listens for ICMP echo message reply messages from its intended host. Using the -R option with ping enables the record route feature. If this option is used ping will set the record route (RR) in the outgoing ICMP IP datagram


The "traceroute" program uses ICMP messaging and the time to live (TTL) field in the IP header. It works by sending a packet to the intended host with a TTL value of 1. The first router will send back the ICMP "time exceeded" message to the sending host. Then the traceroute program will send a message with a TTL of 2, then 3, etc. This way it will get information about each router using the information received in the ICMP packets. To get information about the receiving host, the message is sent to a port that is not likely to be serviced by that host. A ICMP "port unreachable" error message is generated and sent back.


Some telnet command codes and their meanings

Command CodeDescription
237SUSP - Suspend the current process
238ABORT - Abort process
239EOR - End of record
240SE - Suboption end
241NOP - No operation
242DM - Data Mark
243BRK - Break
244IP - Interrupt process
245AO - Abort output
246AYT - Are you there
247EC - Escape character
248EL - Erase Line
249GA - Go ahead
250SB - Suboption begin
251WILL - Sender wants to enable option / Receiver says OK
252WONT - Sender wants to disable option / Receiver says not OK
253DO - Sender wants receiver to enable option / Receiver says OK
254DONT - Sender wants receiver to disable option / Receiver says not OK

On items 251 through 254 above, a third byte specifies options as follows:

3Supress go ahead858
6Timing Mark860
24Terminal type1091
31Window size1073
32Terminal speed1079
33Remote flow control1372
34Line mode1184
36Environment variables1408