Sunrise telecom XTT 5000, User Manual

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XTT 5000 User's Manual
If we send a series of echo requests and never get any echo replies, then something
is broken: perhaps the network itself is down, or perhaps the computer we are trying
to reach has lost its network connection.
If we send a series of echo requests, but only get replies to some of them, then the
network and remote computer are working, but not very well: some IP packets are
getting lost. Even though TCP can compensate for lost packets, there is a limit to
how much it can do – and every time a packet gets lost TCP has to send it again,
making the overall network slower.
As a rough guide, anything more than about 10% packet loss will break TCP, and
anything more than 1% will tend to make it painfully slow. A well engineered
network should have negligible packet loss.
Each time we send an echo request, we can time how long it takes for the echo reply
to come back. This can tell us something about the quality of the network
connection. For example, if it takes a second for the reply to come back, then there
is a long network delay which probably makes it unusable for some delay-sensitive
applications, such as voice.
Routers
A router is a computer in the core of the network that forwards – routes – packets
from one part of the network to another. It has multiple network interfaces, each
connected to another router. Every time it receives a packet, a router looks at the
destination IP address in the IP header, consults its internal tables to decide what to
do with the packet, and then forwards it on, usually over a different network
interface to a different router.
The backbone of the Internet is made up of thousands of routers, working in
collaboration to forward packets from one to another, until they reach their
destination.
Gateways
A gateway is a router that provides access to the Internet for user computers; it
connects dissimilar networks and passes information between them.. On one side it
has one or more connections to network of routers that make up the internet. On the
other side it connects to individual computers.
In TCP/IP, the default gateway address is the address where the Internet protocol
sends packets destined for remote networks, unless a different route is configured.
Only used for static IP.
There is no fundamental difference between what a gateway does and what a router
does (and the terms are often used interchangeably). They both take in packets on
one interface, and forward them out of another, according to the destination IP
address.
The difference is in their position in the network. Routers live inside the network,
communicating with one another. Gateways live at the edge of the network,
communicating between routers and individual users.
DNS
The DNS (Domain Name System) was created to handle the challenge of both
remembering IP addresses and the fact that computer addresses may change over
time. DNS runs on computers known as Name Servers. They have regularly updated
tables of the names and IP addresses of all known computers on the Internet.
When you type ”r;www.google.com” into the address bar of your web browser, the
first thing that happens is that a DNS request is sent to one of the name servers
saying ”r;what is the IP address of www.google.com?” The name server will reply
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