AFAIK when i turn on my modem, it says: "Hi ISP, i need an IP".
Then, my ISP give it an IP.
How does my ISP identify my modem? by last IP, key, or what?
After i have an IP, i can navigate, but after some hours my modem changes its IP.
2) How is that change done?
I mean, my ISP says: "Hi user modem, there is your new IP"
It gets even funnier if when i turn on my modem, some other modem has the last IP my modem had.
There is a collision. So, my ISP would give my modem another IP, wont it?
I know im talking about technical stuff, but i would like you to explain me in your own words in order not to make it cumbersome.
If technical references arise, maybe just name the concept or leave a link. It would be enough for me.
Thanks you all!
By posting under the tag „dhcp“, you are obviously already assuming that the DHCP protocol is the answer to your questions:
The DHCP Protocol allows the DHCP Server (your ISP) to identify the DHCP client (your modem) by a multitude of information. The most important one is usually the MAC address of your modem. The last IP is also transmitted from client to server along with proprietary information such as the client identifier and others.
The server supplies the IP address along with a lease time. The client will renew the IP address with the DHCP server when the lease time is about to expire. The server decides in the renewal process triggered by the client if the same IP is ok to use further or not.
Restart is not much different from renewal. The DHCP protocol is for that purpose equipped with a broadcast feature so the collision does not really happen, because the client (modem) will ask for an IP address before it uses the old IP.
It is possible that modem and ISP do not use DHCP but the mechanism is probably similar. DHCP is specified in RFC 2131.
Related
I'm currently learning about ARP and L2/L3 networking - would someone be able to help out? If I use an ethernet cable to connect my laptop (A) to another laptop (B), it could use ARP to find out B's MAC address based on B's IP address. Then, any ethernet frames could be sent and accepted by B.
But how does A know B's IP address in the first place? What if there is a switch (L2 device) in between A and B? Does this change the answer? What if there was a router in between (an L3 device)?
Usually, a switch/Router will assign an IP.
The question is unclear -
find out B's MAC address based on B's IP address.
B's MAC address is not based on B's IP.
A MAC is associated to the hardware (but somewhat assignable/cloneable) where as an IP is assigned. Either by DHCP or statically by an admin.
Effectively, you need a switch/router to make what you're defining work.
EDIT:
you can connect two computers direct, but you still need to give each a static IP address so they can find each other - this would likely not be ona network so the IP sub-network doesn't matter.
It still has very little to do with MAC.
If two PC's hand-shake, they'll see each others MAC.
It sends out a broadcast packet to the broadcast MAC address asking "who has the IP address x.x.x.x" which goes to all computers on that broadcast domain. If a computer sees that packet that owns the IP address "x.x.x.x" it will send a unicast reply back to the MAC address of the original requestor with its MAC address which will then be cached for further communication.
It doesn't know the IP unless you tell it. You say "the default gateway is 192.168.1.1" and it will start trying to talk to 192.168.1.1 etc.
If there is a router in between, all traffic to the remote device would go to the MAC of the router and your computer wouldn't be doing any ARP lookups other than that router.
Switches wouldn't matter for this, all they do is make the broadcast domain bigger. With a cable the broadcast domain is just you and the remote device. With a switch, it expands that to all devices connected to the same switch (or VLAN in the switch)
But how does A know B's IP address in the first place?
It doesn't know. At least there's no general method to find out.
The IP address of any resource needs to be supplied by "something else". You need to either provide the address yourself, resolve it from a (also provided) DNS name, read it from a file, ... Alternatively, you need to provide some kind of auto discovery between the nodes (broadcast, multicast, LLDP, ...) or provide a commonly known discovery service, registration server or similar.
What if there is a switch (L2 device) in between A and B?
That changes nothing but excludes link-level discovery (unless it's a "dumb" switch that simply forwards those frames).
What if there was a router in between (an L3 device)?
That eliminates broadcast and multicast from the picture. (Multicast could be routed but that is unlikely for discovery and I won't dive into that.)
I have question on DHCP server in home router. I have founded, that I have same IP address in my notebook for some time. I don't know, how long it last, but it is probably few weeks at least. Will DHCP server ever change my local IP if i will still connecting to that Wi-Fi?
I am asking, because I am working on home automation system and I don't know if i had to keep care about checking if my devices has same IP...Because there will be server, which will remember IP of its clients (lets say ESP8266 modules). Must I periodically check IP address (in my modules) and send new IP to server (in case of change)? Or DHCP server will not change IP address of my modules, connected to that DHCP server? And what about situation when ESP8266 module will disconnect (lets say it will be off for few days) - is it common to get different or same IP from DHCP after connection again?
Thanks!
Simple solution would be reserving IP address for your ESP8266 modules in your DHCP server that they always receive the same IPs.
With regards to your questions:
My routers DHCP server saves clients and keep information about them unless it is deleted explicitly or number of saved clients gets larger than number of addresses available. It serves always the same IP to saved clients. Obviously it depends on the router and might be different in your case.
I'd not check for IPs manually, I'd rather reserve the IPs for the modules as said above.
If your DHCP server stores client information it will give the same IPs to your modules upon reconnect. Otherwise not it won't be necessary the case.
If you use or consider using MQTT server (my preferred way :) ) for your home automation (it has integration in many systems like e.g. Home Assistant) you won't need to care about IP addresses of your modules at all you'd only need to fix IP of the MQTT server.
Another option can be addressing your modules (or/and server) using host name instead of IPs. If you develop n Arduino this GitHub thread might be helpful.
Can I force any device plugged into a particular Ethernet port on a router or managed switch to receive a fixed IP address, without knowing its MAC or anything else? I'm thinking there's a way to do this in dd-wrt, but it doesn't need to be that platform. Would need to be able to run it on a managed switch, or at least something with more than four ports. I'd also need to be able to swap them out quickly, so not wait for lease expiry. Don't need a step-by-step solution, just a bearing would help.
Short answer: no.
Longer answer: Trying to get a certain (reserved) IP address leased to a device on a specific switch port would need to overcome several problems:
The DHCP server doesn't know the switch port of the DHCP client asking for an offer.
The DHCP server doesn't know when a device has disconnected and its lease is potentially free again.
The closest I can imagine is a separate VLAN per switch port with a single IP address in the VLAN scope and a very short lease time. The DHCP server would also need to be directly connected with each VLAN, alternatively you could use a DHCP agent or relaying on the switch.
I basically have used the ethernet connection successfully with Microcontrollers to control real world aspects,The dilemma is of IP address I mean how can 192.168.0.155 be accessed from around the world without any specific MAC ID attached to it, I wanted to implement the same networking usage with a GSM module eliinating wires but that doesn't have MAC addressing so TCP connection from it towards a IP client won't reach?
I am not sure how you did your 'real world control' with Ethernet before, but the MAC address does not 'live' past the next recipient (which means after the next switch) so do not use it for anything.
Your 192.168.x.x address is private, so it will never appear beyond your internet gateway (i.e. your GSM modem if you used GPRS), and it cannot be used directly for anything on the internet. It is not easy to briefly explain here how the internet routing works, but you should either use port forwarding towards your private IP address given to you by hour DHCP server (in which you have to initiate request from the client so that other side would not the IP address and the port), or you have to either have fixed public IP address or you could use some dynamic DNS service (like http://www.noip.com/), or you can update data from the client on some server over which you have control (like if you updated data manually from your PC).
All summed up, it is not very clear what you are trying to achieve, but I would say you need to check out some things about internet routing before you proceed.
This question used to raise from the time i was studying my networking course. Suppose I am chatting with some other person on the g-talk and if I capture the packets then whose ip address it would be whether it would be the ip address of the person whom i am chatting with or of the relay server in between.
Is there any way i can get the ip address of the person whom i am chatting with ??
Generally - no. Since there is no way to establish direct connection between ANY two hosts (two peers both behind NAT will not be able to connect directly), all communications should occur through relay server, so any packets you capture will be from and to relay server.
But GTalk developers MAY implement a feature to connect two chatting peers directly (if possible). In that case you will be able get the other person's IP address.
Also relay server may simply report other person's IP address.