Multitech RF500S Uživatelský manuál stáhnout pdf (Strana 23)

6.3 TCP/IP

The LAN uses the Internet Protocol (IP) to connect local devices. Using the same communication protocol

for the LAN and the Internet simplifies configuration and management of the LAN. IP is the mechanism

used to deliver a packet of data from one computer to another. TCP stands for Transmission Control

Protocol. IP is an unreliable delivery mechanism it launches packets to the Internet; they may arrive out of

order and not at all. TCP orders the incoming packets and requests retransmission of any that are missing.

When an application creates a TCP/IP connection the receiver sees the same data stream that was

transmitted.

A simpler mechanism, UDP/IP User Datagram Protocol, is used when end-to-end synchronization is not

required. UDP is a connectionless protocol. The transmitting station simply casts the packets out to the

Internet. Each packet is dealt with individually. UDP is often used with multimedia. If a packet is lost it

cannot be retransmitted in time so the receiver has to fake the missing information.

ICMP Internet Control Message Control Protocol handles control function.

6.4 IP Address

Each IP device (node) must have an address. Addresses can be assigned, statically, automatically by DHCP

(Dynamic Host Control Protocol) or automatically by the client when DHCP is not present, AutoIP.

Traditionally the system administrator manually configured each device with an address. This was labor

intensive and error prone. DHCP simplified the task by centralizing address assignment. The down side is a

DHCP server is required to allocate addresses. Recently the DHCP protocol has been extended to allow

automatic configuration if the host cannot find a DHCP server. In that case the device assigns itself an

address after failing to find a DHCP server and automatically determining the address is not in use. This is

convenient for small LANs that use IP and do not have access to a DHCP server. This occurs most

commonly when two PC’s are directly connected. Most Internet sharing packages and hardware access

devices implement a DHCP server.

The current version of IP is version 4 each node is assigned a 32-bit address, so the maximum population of

the Internet is 4 billion devices. This has been recognized as a serious limitation for some time and a new

version of IP version 6 expands the address space to 128 bits. This is a truly gigantic number. If IPv6

addresses were uniformly distributed over the Earth it would result in thousands of addresses per square

foot. Several techniques are discussed to conserve the limited IPv4 address space.

6.4.1 Dotted-Decimal Notation

Internet addresses are expressed in dotted decimal notation, four decimal numbers separated by periods,

nnn.nnn.nnn.nnn. The 32-bit address is divided into four 8-bit fields called octets. Each field has a range of

0-255. The smallest address is 0.0.0.0 and the largest 255.255.255.255.

6.4.2 Subnet

IP addresses consist of three components, the Network-Prefix, Subnet-Number and the Host Number. The

purpose of Subnetting is to allow IP addresses to be assigned efficiently and simplify routing.

For our purposes all the computers on the network must be on the same subnet. For example our network

allows up to 254 hosts (computers) the subnet is 255.255.255.0, also called a /24 subnet because the first 24

bits are fixed.

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Multitech RF500S Uživatelský manuál Strana 23