There are myriad types of problems you may encounter on Internet systems. However, this lesson focuses on a few specific errors and how to correct them.
If you run into problems that seem related to IP or host file configuration errors, consider some of these possible solutions.
|Local network is running but web server cannot reach a site outside the network
||Router failure at the ISP or an incorrectly set gateway on the local network
||Call the ISP to determine outages. If none, check the server's gateway and IP addressing
|You just moved a server to anew IP address. Some user can access it at the same URL, but other cannot
||The DNS entry needs to be updated.
||Update the DNS tables on all DNS used by all nodes on network
| A domain has just been moved to a DNS server. Users complain that they can access Web sites, but their email is bouncing.
||The MX record at the DNS server may be set incorrectly or the mail server may be down.
||Check the MX records for proper addressing, and check the mail server for functionality.
|Some users get a 403 Forbidden error message when trying to execute a CGI script. Others do not have this problem
||The IP filter on the web server may have set to deny access to specific IP addresses or a range of IP addresses
||The site is password and/or username protected, so you need to try the password and/or username again.
If you do not have one, contact site's webmaster about eligibility requirements
The fields of the IP packet are as follows:
- Version:Indicates the version of this IP datagram.
- IP Header Length (IHL):Indicates the datagram header length in 32-bit words.
- Type-of-Service:Specifies how a particular upper-layer protocol would like the current datagram to be handled. Datagrams can be assigned various levels of importance using this field.
Today this field is used primarily to provide quality of service (QoS) capabilities to TCP/IP for applications requiring predictable bandwidth or delay.
RFC 2474 describes a method by which the TOS field is replaced by a DS field that is used to provide differentiated services (DiffServ) on networks. This field is split into two parts and the first 6 bits are used for the DSCP codepoint, which is used to differentiate traffic. The last 2 bits, or CU, are ignored by DiffServ-compliant nodes.
- Total Length:Specifies the length of the entire IP packet, including data and header, in bytes.
- Identification:Consists of an integer identifying this datagram. This field is used to help piece together datagram fragments.
- Flags: Consists of 3 bits, of which the low-order 2 bits control fragmentation. One bit specifies whether the packet can be fragmented; the second bit specifies whether the packet is the last fragment in a series of fragmented packets.
- Time-to-Live: Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This keeps packets from looping endlessly.
- Protocol: Indicates which upper-layer protocol receives incoming packets after IP processing is complete.
- Header Checksum: Helps ensure IP header integrity.
- Source Address: Specifies the sending node.
- Destination Address: Specifies the receiving node.
- Options: Allows IP to support various options, such as security.
- Data:Contains upper-layer information.