> For the complete documentation index, see [llms.txt](https://amartyushov.gitbook.io/tech/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://amartyushov.gitbook.io/tech/os-components/network/tcp-ip-model/ip-address-in-action.md).
# IP address in action
## LAN
* Network ID = 202.120.10.0
* Host ID = e.g. 42, or 164 (in this case)
* IP = Network ID + Host ID
In order to interconnect LANs we need a router (interconnection happens through router).
The router interface that connects LAN to the router is called **default gateway**
* must have same Network ID
* usually has the lowest host address in LAN
* in above case it will have IP 202.120.10.1
Ranges of addresses that are designed as private IP addresses:
127.0.0.0/8 subnet is reserved as loopback address
### Subnet mask
Every TCP/IP host needs to know whether the destination IP is local or long distance. Subnet mask helps here.
Short notation for subnet masks:
* 11111111.11111111.11111111.00000000 = /24 (24 ones)
* 11111111.11111111.00000000.00000000 = /16 (16 ones)
Below picture shows how Subnet mask helps to understand that Computer A and B are in the same LAN.
If we need to send a request to another LAN:
* send ARP request to realise MAC address of default gateway
* IP packet will be sent to default gateway
## CIDR (Classless inter-domain routing)
CIDR and subnetting are virtually the same thing.
Subnetting enables much more efficient use of IP addresses compared to class blocks.
example of subnet calculation
there is a cafe with network ID 192.168.4.0/24 and you need to split it to subnets.
* start with a given subnet mask and mote it to the right until you have a number of subnets you need
* forget the dots (**never try to subnet without first converting to binary**)
How many hosts we have on /24 subnet? 2^8 - 2 = 254
How many hosts we have on /26 subnet? 32 digits - 26 = 6 => you have 6 zeroes left after 26 ones => 2^6 - 2 = 62 hosts.
Original network ID: 192.168.4.0/24 translates to this binary (grey below):
By taking 2 more bits (red ones) for subnetting we can have 2^2 new subnets (we get 4 new Network ids).
## DHCP (Dynamic IP addressing)
```
DORA (DHCP four-way handshake):
(Discover) [Client] -> {broadcast} -> is there DHCP server here
(Offer) [DHCP Server] -> [Client] : I am, here is your IP address, subnet mask, default gateway
(Request) [Client] -> [DHCP Server] : I am accepting your information
(Acknowledgmnet) [DHCP Server] -> [Client] : ok, I save you IP and MAC to my db
------ in around 5-6 days:
(Request) [Client] -> [DHCP Server] : can I use my setting?
(Acknowledgmnet) [DHCP Server] -> [Client] : I know you, I look up your data in db and give it back
```
Usually routers are blocking broadcasting traffic, but what if in big company there are many subnets and just one DHCP server for convenience =>
### DHCP relay
DHCP relay enables DHCP traffic (broadcast) to cross routers.
