02 TCP/IP Basics | Senalyst

IP Addressing and Binary

An IP address is represented as four values separated by three dots (172.0.0.1)
A real IP address is 32 ones and zeros (10101100.00000000.00000000.00000001)
Dots don’t exist in IP addresses (10101100 00000000 00000000 00000001)
Each octet has 2^8 (256) combinations (0 ~ 255)

ARP resolves MAC addresses from IP addresses
ARP is used to construct an entire frame by finding a destination MAC address, which is going to be attached into IP packet(dest IP srce IP dest Port srce Port Data)
arp -a command shows ARP cache

IANA(Internet Assigned Numbers Authority) keeps track of all of the IP addresses and passes them out to those who need them
In fact, IANA passes out chunks of IP addresses to RIRs(Regional Internet Registries). e.g. ARIN: the American Registry for Internet Numbers
RIRs pass out big chunks of IP addresses to ISPs(Internet Service Providers)
Class licenses
- Class A : 0-126/8, 2^24 (16,777,216) IP addresses, usually large organization like Comcast
- Class B : 128-191/16, 2^16 (65,536) IP addresses
- Class C : 192-223/24, 2^8 (256) IP addresses
- Subnets don’t have to be on the dots!

Network ID: A part of the network numbering system that has to be identical for every computer on a particular network
Host ID: A part following the network ID that changes for every individual computer in a network
Subnet Mask: a string of ones followed by a certain number of zeros
232.25.208.0/255.255.255.0
- 232.25.208 -> Network ID
- 0 -> Host ID
- 255.255.255.0 -> Subnet Mask, simply say 24 in this case
The host uses the subnet mask to know if the destination is on the local network or a remote network
Each host knows the default gateway so that it can forward traffic to remote networks

/24 = 254 hosts (=2^8-2), the network ID is up to the third octet
/25 = 126 hosts (=2^7-2), the network ID is up to the first number in the last octet
/26 = 62 hosts (=2^6-2), the network ID is up to the second number in the last octet
/27 = 30 hosts (=2^5-2), the network ID is up to the third number in the last octet
/28 = 14 hosts (=2^4-2), the network ID is up to the fourth number in the last octet
/29 = 6 hosts (=2^3-2), the network ID is up to the fifth number in the last octet
/30 = 2 hosts (=2^2-2), the network ID is up to the sixth number in the last octet
/31 = 0 hosts (=2^1-2), the network ID is up to the seventh number in the last octet

Three IP address settings for a host - IP address / subnet mask / a default gateway
When your computer first boots up, it doesn’t have any IP information at all
A DHCP(Dynamic Host Configuration Protocol) server supplies them to the computer
A DHCP server is usually located in a separate server in a local network or built in a router attached to the local network
DHCP process
- When your computer first boots up, he will beging sending out a broadcast called a DHCP Discover (a broadcast on the MAC address of all F’s to all computers in the local networ, looking for a DHCP server)
- A DHCP server, upon receiving the broadcast, sends unicast traffic back with a DHCP Offer
  - A DHCP Offer has all three essential information(IP addr, subnet mask, default gateway) and other stuffs
- Your computer, upon receiving a DHCP Offer, sends a DHCP request back to the DHCP server
  - Basically telling the DHCP server that I am going to take and use this information that you are giving me
- Once the DHCP server hears that, he sends a DHCP acknowledgment
- The DHCP server will store all of this information and keep track of all of the different clients that are using DHCP
Each broadcast domain must have only one DHCP server
Every modern operating system compes with DHCP enabled by default
DHCP Relay enables a single DHCP server to service more than one broadcast domain

APIPA (Automatic Private IP Addressing)
- built into all of your DHCP clients and designed as a fallback if you can’t find a DHCP server
- If your DHCP server IP address starts with 169.254(APIPA address), telling that your client cannot connect to a DHCP server
If you get an APIPA address, check to see if you are connected to a DHCP server
If you are connected to a DHCP server and still get an APIPA address, make sure the DHCP server is working
If you get an IP address other than your correct network ID, you may have a rouge DHCP server

Private IP addresses - only for serving a local network
- 10.x.x.x
- 172.16.x.x - 172.31.x.x
- 192.168.x.x
Loop back - talk to yourself
- 172.0.0.1(IPv4)/::1(IPv6)
- the request goes out and come back in to test a network card (old days)
- used to test their own internal circuitry (these days)
- In reality, 127.x.x.x will always loop back
APIPA
- 169.254.x.x
- If a DHCP server’s down APIPA will be given so at least the host can talk locally
- You won’t be able to get on the internet but you’d be able to share folders and files

Duplicate IP addresses
- caused by a rouge DHCP server
- The system is going to turn off your static IP address and try to set itself to DHCP
Duplicate MAC addresses
- happens when you’re working with virtual machines
- For VM’s MAC address, I can type in whatever I want, accidently leading to the exact same MAC address for two completely different VMs
Incorrect gateway
- You can’t get out of your local network
- You still can share files and folders within the local network
- Remember that your gateway is your router’s IP address
Incorrect subnet mask
- Keep in mind that all of the computers within the same broadcast domain will always have the same subnet mask. No exception
- A host with subnet mask of 255.255.0.0 is able to ping another host with 255.255.255.0, but the opposite direction is impossible
Expired IP address
- When a computer goes to a DHCP server it gets a lease. The period is often 8 days
- Once it passes half of the lease period, the computer tries to re-establish the lease by interacting with the DHCP server
- If the DHCP server dies, you have expired your DHCP lease
- In Windows, the computer will keep that IP address at least through the time of the DHCP lease
- Or the current IP address will be switched into APIPA(169.254.x.x)