The Internet: routing and name resolution

The topology of the internet is a mix of [blank_start]tree[blank_end], [blank_start]mesh[blank_end] and fully [blank_start]connected[blank_end] networks.

For example, [blank_start]Heanet[blank_end] connects most colleges in Ireland while [blank_start]Janetbackbone[blank_end] connects most colleges in the UK.

They key aim of the internet is that it should ideally be possible to reach any [blank_start]host[blank_end] on it from [blank_start]any other[blank_end].

For routing to work, IP addresses must be [blank_start]structured[blank_end] (i.e. having [blank_start]netmasks[blank_end]. This is comparable to the first letter of room numbers). [blank_start]Routing[blank_end] tables ([blank_start]rules[blank_end]) must be established.

The [blank_start]default gateway[blank_end] is the simplest routing table of all: it sends all traffic to a specific [blank_start]router[blank_end] on the [blank_start]LAN[blank_end], and lets the [blank_start]router[blank_end] deal with it from there.

The internet and the world wide web are not the same thing! The web is a particular [blank_start]protocol[blank_end] that runs over the internet.

From early stages in its development, users preferred to refer to other hosts by [blank_start]name[blank_end] rather than [blank_start]IP addresses[blank_end]. This means we need a way to resolve a [blank_start]host[blank_end] name into an [blank_start]IP address[blank_end].

When a host needs to be contacted by [blank_start]name[blank_end], the [blank_start]name resolution[blank_end] process is in order: firstly, it must be checked if the [blank_start]hostname[blank_end]/IP mapping is [blank_start]cached[blank_end] from before.

The [blank_start]hosts file[blank_end] is read on [blank_start]disk[blank_end] to see if there's a matching entry.

The configured [blank_start]DNS server[blank_end] is then checked for the IP.

Once the name is known, packets are either [blank_start]routed[blank_end] or [blank_start]ARP[blank_end] is done and packets are sent directly as before. On most operating systems, the [blank_start]host[blank_end] or [blank_start]nslookup[blank_end] commands can be used to directly query an IP address for a hostname.

What does the acronym DNS stand for: [blank_start]Domain name system[blank_end].

The [blank_start]DNS[blank_end] provides a way to look up any host on the internet by it's fully qualified domain name. Only the [blank_start]IP address[blank_end] of a DNS server is needed, which is usually on our [blank_start]local network[blank_end].

So that the DNS system could be [blank_start]federated[blank_end], it was decided that each host would have a unique [blank_start]hostname[blank_end] within a given [blank_start]domain[blank_end] name. For example: printstaff01.ad.dkit.ie means that the hostname is [blank_start]printstaff01[blank_end] and the domain name is [blank_start]ad.dkit.ie[blank_end]

The acronym TLD stands for [blank_start]Top Level Domain[blank_end].

An example of a TLD is [blank_start].ie[blank_end] or [blank_start].com[blank_end]

Using the previous example of printstaff01.ad.dkit.ie, [blank_start]ad.dkit.ie[blank_end] is a subdomain of [blank_start]dkit.ie[blank_end] which is a subdomain of the [blank_start].ie[blank_end] domain.

When a local DNS server on a network is asked by a client to resolve a [blank_start]hostname[blank_end], it go [blank_start]recursively[blank_end] through the subdomains, starting with the [blank_start]root[blank_end] and the [blank_start]TLD[blank_end] on the right. For each subdomain, it will get the [blank_start]IP[blank_end] of the [blank_start]authoritative[blank_end] DNS server for that subdomain, which it will then [blank_start]query in turn[blank_end].

Ports allow us to identify to a [blank_start]remote host[blank_end] what service a packet should be sent to when it gets there. Each packet is tagged with the port so that the receiving computer knows [blank_start]what to do with it[blank_end].

Common internet port numbers include [blank_start]53[blank_end] for DNS lookup, [blank_start]3389[blank_end] for Windows Remote Desktop and [blank_start]22[blank_end] for Secure Shell.