Multitech RF500S Uživatelský manuál stáhnout pdf (Strana 24)

6.4.3 Port Number

A single computer may be connected to multiple hosts over the Internet. How does the computer know how

to deliver each packet? For example, while writing this paper my mail program is checking e-mail, and I’m

listening to a Real Audio radio program. Each IP packet includes a port number. Port numbers are 16 bit

values that range from 0-65,535. For example when you enter a URL into you web browser to access a

World Wide Web site the browser automatically uses port 80. The low port numbers 0-1023 are called the

well-known ports; they are assigned by IANA the Internet Assigned Number Authority when a particular

service is defined. Software uses that port to make initial contact. After the connection is established the

high numbered ports are used.

6.5 Private Addresses

The Internet Assigned Number Authority assigns Internet addresses. This is the entity that assigned the

addresses used by your ISP. IANA allocated three blocks of private addresses that are guaranteed not to be

used on the Internet RFC 1918. The private addresses are ideal for a small LAN. Devices on the LAN are

assigned from the pool of private addresses. This eliminates the need for coordination of the IP addresses

used on the LAN with those used on the Internet at large. When a computer on the LAN needs to access the

Internet the gateway router uses a technique called Network Address Translation (NAT) to convert the

private IP addresses to the public address assigned by the ISP.

Excerpt from IETF RFC 1918 Address Allocation for Private Internets

The Internet Assigned Numbers Authority (IANA) has reserved the

following three blocks of the IP address space for private

Internets:

10.0.0.0 - 10.255.255.255 (10/8 prefix)

172.16.0.0 - 172.31.255.255 (172.16/12 prefix)

192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

We will refer to the first block as "24-bit block", the second as

"20-bit block", and to the third as "16-bit" block. Note that (in

pre-CIDR notation) the first block is nothing but a single class A

network number, while the second block is a set of 16 contiguous

class B network numbers, and third block is a set of 256

contiguous class C network numbers.

An enterprise that decides to use IP addresses out of the address

space defined in this document can do so without any coordination

with IANA or an Internet registry. The address space can thus be

used by many enterprises. Addresses within this private address

space will only be unique within the enterprise, or the set of

enterprises, which choose to cooperate over this space so they may

communicate with each other in their own private Internet.

In our implementation DHCP is built into the Multitech broadband router. We use Class C private

addresses in the range of 192.168.2.x this allows up to 254 hosts on the LAN. The IP address of the NAT

router is statically assigned as 192.168.2.1. The DHCP server in the router then assigns an IP address to

each client from the pool of remaining addresses.

Some devices work better with a static address. Our local web and print server are assigned static address

so the address is constant. An option in the router forces the router to always issue the same address to a

given device.

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