How does DNS work step by step

Client enters a domain name (www.domainname.com) into his browser
The browser contacts the Client's ISP for the IP address of the domain name
The ISP first tries to answer by itself using "cached" data.
If the answer is found it is returned. Since the ISP isn't in charge of the DNS, and is just acting as a "dns relay", the answer is marked "non-authoritative"
If the answer isn't found, or it's too old (past the TTL), then the ISP DNS contacts the nameservers for the domain directly for the answer.
If the nameservers are not known, the ISP's looks for the information at the 'root servers', or 'registry servers'. For com/net/org, these start with a.gtld-servers.net.
NOTE: The 'whois' information is never used for DNS, and is often misleading and inaccurate

Diffrence between bridging and Routing

both Bridging and routing are used for connecting two or more networks


when bridge is installed it gathers all the packets from one network 
then repeat them on other network.
it have features like Broadcast filtering
it takes place in data link layer of OSI model

Routers do packets exchange
they work in network layer of OSI model
they provide various ways of filtering
like path control and traffic control.

What is MD5/MD5SUM?

The MD5 checksum for a file is a 128-bit value, something like a fingerprint of the file. There is a very small possibility of getting two identical checksums of two different files. This feature can be useful both for comparing the files and their integrity control.

Let us imagine a situation that will help you to understand how the checksum works.

Alice and Bob have two similar huge files. How do we know that they are different without sending them to each other? We simply have to calculate the checksums of these files and compare them.

The RFC 1321 describes MD5 checksum (MD5 message-digest) as:

The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.