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8-port Gigabit Ethernet Switch User's Guide
IP removes the IP header. It looks at the IP protocol field. Since the protocol type is TCP, it passes the packet to TCP. TCP
now looks at the sequence number and uses it to recombine the packets in the correct order.
The Domain Name System
Most network software uses a 32 bit IP address to identify network devices and computers on the network. User's
generally prefer to use text names for network nodes. So, a database is established that contains the text names and the
corresponding IP addresses. The network software can then use the text name to look up an IP address. This database is
located in a Domain Name System (DNS) server.
DNS is used to associate IP addresses to text names throughout the Internet. The same method has been adapted for use
within intranets.
Resolving Domain Names
To resolve a domain name, a query is sent to a DNS server. This server then checks if the name is in its database. If it is,
the DNS server translates the text name into an IP address and sends the answer back by appending the answer to the
original query. If the DNS server can not resolve the name, it checks to see what type of resolution is specified in the query.
The query can specify a complete translation (recursive resolution). In this case, the DNS server contacts another DNS
server and forwards the query. If the query specifies iterative resolution, the DNS server replies that it cannot resolve the
name and specifies the DNS server that should be contacted next to resolve the name.
Each client must be able to contact at least one DNS server, and each DNS server must be able to contact at least one root
DNS server.
The IP address of a local DNS server is often supplied by a DHCP or BOOTP server.
DHCP Servers
The Dynamic Host Configuration Protocol (DHCP) is used to dynamically assign a TCP/IP network configuration to
network devices and computers on the network. It also ensures that IP address conflicts do not occur.
IP addresses are assigned from a pool of free addresses. Each IP address assigned has a 'lease' and a 'lease expiration
period'. The lease must be periodically renewed. If the lease is expires, the IP address is returned to the pool of available IP
addresses.
Usually, it is a network policy to assign the same IP address to a given network device or computer each time.
If the IP address lease expires, the network device sends a message to the DHCP server requesting a lease renewal. The
DHCP server can send an acknowledgement containing a new lease and updated configuration information.
If an IP address lease cannot be renewed, the network device or computer sends a request to all local DHCP servers
attempting to renew the lease. If the DHCP returns a negative acknowledgement, the network device must release its
TCP/IP configuration and reinitialize.
When a new TCP/IP configuration is received from a DHCP server, the network device checks for a possible IP address
conflict by sending an Address Resolution Protocol (ARP) request that contains its new IP address.
Routing
The task of determining how a packet should get from a source to a destination is referred to as routing.
IP assumes that the networks a packet would be sent across are connected by gateways (also called routers).
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Troubleshooting
