Basics
File identification
All files are given a hash value. This hash is a combination of numbers and letters to uniquely identify the file. Numerous filenames may be associated with a file, but this does not change anything about file’s hash value. This allows each user to find all sources to a particular file no matter what file name each user has given the file.
In addition, the files are broken into 9.28 MB of parts of data. Each part is also given a hash value. For example a 600 MB file would contain 65 parts. Each part is then given a hash value. Then the file hash is created from these part hashes to be used in the networks.

Identifying other clients
Like the file hash, each user in the network gets a unique and permanent user hash. This user identification is highly secured by a public / private key handshake to prevent misuse.

Downloading Data
It is important to understand that the actual downloading in eMule is not affected by the choice of the network. The network topology is only related to searching for files and finding clients that are sources to a file.
Once a source has been found, your client contacts it. The source then reserves a queue place for that specific download. When you reach the first queue place after a certain waiting time you are entitled for receiving data.

Classic server based eD2k
Connecting to the network
The key to this network is the eD2k server. Each client must to be connected to a server to enter the network.
When connecting your client to a server, the server checks to see if other clients can freely connect to your client. If yes, the server assigns your client a so-called high ID. If communication is blocked, the server assigns your client a low ID.
After the ID is assigned, eMule will send a list of all shared files to the server. The server adds the filenames and hash values you sent to its database.

Searching for files
Once connected to the network, the client can search for keywords in filenames. A search can either be local or global. If it’s a local search (searches only the server you are connected to), searches are quicker but will have fewer results. If the search is a global search (searches all the servers within the network), it will take longer but have more results. Each server looks up the keyword in its local database and returns any file names (with the hash value) that matches the keyword.

Finding sources for files
Downloads can be added by eMule’s search function or a special eD2k link format offered on many websites.
Once they are in the Download list, eMule first queries the local (connected) server then all other servers in the network for sources to that particular download. The server looks up the file’s hash value in its database and returns the clients it knows for having it.
Sources are other clients who have at least downloaded one entire part (9.28 MB) of the file matching the hash.

Kademlia serverless network
Connecting to the network
The only thing needed to connect to this network is the IP and port of any eMule client already connected. This is called a Boot Strap.
Once a client is in the network, the client then requests for other clients to determine if it can be contacted freely. This process is very similar to the HighID/LowID check on the servers. If you can be freely contacted, you are assigned an ID (similar to a HighID) and given an open status. If you are not freely contacted, you are given a firewalled status. From version v.44a on, the Kademlia network supports a Buddy for firewalled users. Buddies are other Kademlia clients who have status open and work as a relay for connections, that the firewalled user cannot manage.

Searching in Kademlia
In this network it does not matter what you search for. Be it a search for filenames, for sources of a download or for other users, all work pretty much the same.
There are no servers to keep track of clients and the files they share so it has to be done by each participating client in the network – in essence, every client is also a small server.
Since every client is identified by a unique hash value, the idea of Kademlia is to associate a certain “responsibility” based on this hash. Each client in the Kademlia network works as a server for certain keywords or sources. The client’s hash determines the specific keywords or sources.
So the goal of any kind of search is to find those clients that have the responsibility for the current search topic. This is accomplished by a complex calculation of the possible distance to the target client by asking other clients for the shortest route to it.

Summary
Both networks have a totally different concept of achieving the same: Searching for files and finding sources to a file. The main goal of the Kademlia network is to be independent of servers and improve scalability. Servers can only handle a certain amount of users and should a large server go down the network is severely handicapped.
Kademlia is self-organising and tunes itself for best possible performance depending on the number of users and their connection qualities. Therefore, it is more resistance to a large-scale network loss.