Chapter Summary | HP Certified: HP-UX System Administration

TCP/IP is the most widely used and de facto protocol of the Internet. It is a five-layer protocol. The first four layers of the protocol provide the same functionality as defined by the OSI model. The fifth layer combines the upper three layers of the OSI model. TCP/IP layers are listed here.

physical layer
data link layer
network or IP layer
transport or TCP layer
application layer

Unique network addresses known as IP addresses are assigned in the IP layer of the protocol stack. An IP address is a 32-bits-long number consisting of four octets of 8 bits each. An octet is written as a decimal number from 0 to 255. While writing an IP address, the four octets are separated by dot symbols.

Each IP address has two parts , a network part and a host part. The network part is common to all hosts on a network. Netmasks are used to separate the network and host parts of an IP address. Depending on the length of the network part, IP networks are divided into different classes. In class A networks, the network part is 8 bits, and the remaining 24 bits are used for the host part. In class B networks, 16 bits are used for the network part, and in class C networks, 24 bits are used in the network part. IP addresses start in class A networks with a 0 bit, in class B networks with a 10-bit pattern, and in class C networks with a 110-bit pattern.

If you have an isolated network, you can select a network address of your own choice. However, if you want to connect your network to a public network like the Internet, you have to get IP addresses from the network registration body for your geographical location. When you connect two or more networks, routes are established for data transfer among these networks. A default route is defined to transfer data through a particular router for which no other route is available. A route may be a static route or a dynamic route. HP-UX supports static as well as a number of dynamic routing protocols.

Network communication on the physical media takes place using physical addresses instead of IP addresses. For this you have to map IP addresses to physical or MAC addresses. Address Resolution Protocol (ARP) is used for this purpose. The Reverse Address Resolution Protocol (RARP) does the reverse job. The mapping of IP addresses to MAC addresses is kept in the ARP cache, which is maintained in memory by the HP-UX kernel. The arp command can be used to display and manage the ARP cache. Another protocol, known as Internet Control Message Protocol (ICMP), is used to monitor and troubleshoot TCP/IP networks.

TCP and UDP protocols are transport layer protocols. TCP is used for reliable, connection-oriented data transfer, and UDP is used for unreliable, connection-less data transfer. TCP uses a window and acknowledgment mechanism to ensure that data are delivered without any error. Since UDP has no acknowledgment mechanism, it is faster than TCP. When multiple applications on the same host are communicating over the network, ports and sockets are used at the transport layer level to route data packets to different applications. A port is a number assigned to all applications. Standard port numbers are present in the /etc/services file. A socket is a combination of an IP address and a port number.

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