Saturday, January 4th

Differences between inner class and nested static class in Java

Both static and non static nested class or Inner needs to declare inside enclosing class in Java and that’s why they are collectively known as nested classes  but they have couple of differences as shown below:
1) First and most important difference between Inner class and nested static class is that Inner class require instance of outer class for initialization and they are always associated with instance of enclosing class. On the other hand nested static class is not associated with any instance of enclosing class.
2) Another difference between Inner class and nested static class is that later uses static keyword in there class declaration, which means they are static member of class and can be accessed like any other static member of class.
3) Nested static class can be imported using static import in Java.
4) One last difference between Inner class and nested static class is that later is more convenient and should be preferred over Inner class while declaring member classes.

How does DHCP work ?

Have you asked this question to yourself ? Here a quick, not exhaustive answer (thanks to this)

Schema of a typical DHCP session

Client DISCOVERY (broadcast) –>
<– Server OFFER (unicast)
Client REQUEST (broadcast) –>
<– Server ACKNOWLEDGE (unicast)

DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.
DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.
After the client obtained an IP address, the client may start an address resolution query to prevent IP conflicts caused by address poll overlapping of DHCP servers.

DHCP discovery

The client broadcasts on the local physical subnet to find available servers. A client can also request its last-known IP address. If the client is still in a network where this IP is valid, the server might grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP immediately. A non-authoritative server simply ignores the request, leading to an implementation dependent time out for the client to give up on the request and ask for a new IP.

DHCP offers

When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and sending a DHCPOFFER message across the network to the client. This message contains the client’s MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.The server determines the configuration, based on the client’s hardware address as specified in the CHADDR field.

DHCP requests

When the client PC receives an IP lease offer, it must tell all the other DHCP servers that it has accepted an offer. To do this, the client broadcasts a DHCPREQUEST message containing the IP address of the server that made the offer. When the other DHCP servers receive this message, they withdraw any offers that they might have made to the client. They then return the address that they had reserved for the client back to the pool of valid addresses that they can offer to another computer. Any number of DHCP servers can respond to an IP lease request, but the client can only accept one offer per network interface card.

DHCP acknowledgement

When the DHCP server receives the DHCPREQUEST message from the client, it initiates the final phase of the configuration process. This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the TCP/IP configuration process is complete.
The server acknowledges the request and sends the acknowledgement to the client. The system as a whole expects the client to configure its network interface with the supplied options.

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