Why do ip address changes time to time?

[kingston896]

Basically there are not enough IP addresses to go round everyone who needs one so dynamic IP is used. Your dynamic ip comes as my ip after several days?

 

[Barman58]

You would never be allocated my IP, India has it's own ISPs each with their own set of IPs - My ISP has a different set completely and only allocates to their customers in the UK

Try some basic IT theory ...

6 Basics Things to Know About IP Addresses

 

[kingston896]

If they allocate unique private ip for everyone why can't they allocate static ip as publlic ip?

 

[Barman58]

They can, if they want to, and you can afford to pay the premium price, If you need a fixed IP for some purpose you have to go to a supplier that offers the service and pay for it in addition to normal cost, If it's for a web site it's cheaper (though not cheap ), to go to a Specialist hosting company that will handle everything for you. If you want to run a game server at home then you can find a provider to supply a fixed IP for home use although this will be limited to bandwidth and relatively expensive.

As for why they do things this way, it's simple economics, they have to outlay cost to provide a different service to what 99% of users need, so they charge for it,and they charge as much as users are prepared to pay

 

[Alejandro85]

If they allocate unique private ip for everyone why can't they allocate static ip as publlic ip?

Because more often than not (99.99% of cases) fixed vs dynamic IP is irrelevant, so ISPs just chose what's easier for both the ISP and the user and give access to their DHCP server to assign an IP address for you automatically. There is no reason for them to do otherwise, and lots of reasons to not to do it.

So, unless you specifically ask for a fixed IP, you won't be provided one. Neither there is need for you to ask for one, unless you have a specific reason you know to require one. More often than not, the only thing that really matters is that things "just work". The specific address is not really relevant.

They may allocate unique "private" IPs, but consider that those "unique" are only within a specific subnet within the ISP network, not unique though the whole internet. They can use NAT internally to provide multiple internal segments, and even use NAT for their final hop into the world-wide internet. There are some extreme cases that whole countries use a single public IP, NATed internally for every user to a private IP, which is almost always, dynamic.