Post

IP address

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By So Hee Park 9 min read

https://soheeparklee.github.io/posts/chapter5_IPadress/

✅ IP address

unique address for network interface
network address ➕ host address

  • network interface: network communication point
  • host and router

✅ Working of IP address

  1. Device request Internet Service Provider for acess to network
  2. ISP assign device IP address from available range
  3. Internet activity goes through ISP, route activity back to you with IP address

✅ Type of IP address

✔️ IPv4

  • 32 bit binary number
    • 11000001 00100000 11011000 00001001
    • in total 2^32
  • separated in 8 bits, 4 parts
  • each number can be from 0-225
  • written in decimals(10진수) separated with .
    • 193.32.216.9

✔️ IPv6

  • 128 bit binary number
    • in total 2^128
  • separated in 16 bits, 8 parts
  • writeen in hexadecimal(16진수), separated with :
    • 2004:2ba8:13aa:0011:0000:0000:0000:abaa

📌 MTU

MTU: Maximum Transmission Unit
maximum data that a link layer frame can transmit
larget possible link-level frame

Screenshot 2024-09-11 at 18 17 10

1. Different link types, different protocols, different MTUs

  • datagram A: 4000bite(IP header 20byte ➕ payload 3980 byte)
  • datagram A arrives at router
  • datagram A has to be sent through link that has MTU=1500 byte

2. Solution: Fragmentation

Fragmentation: Large IP datagram divided(fragmented) within net

  • one datagram becomes several datagrams
  • reassembled only at final destination

  • IP header bits used to identify, order fragmented datagram

  • datagram A is fragmented into three datagram fragments
    • fragment 1: IP header 20byte ➕ payload 1480 byte
      • ID: x, flag: 1, offset: 0 byte
    • fragment 2: IP header 20byte ➕ payload 1480 byte
      • ID: x, flag: 1, offset: 185 byte(1480/8)
    • fragment 3: IP header 20byte ➕ payload 1020 byte
      • ID: x, flag: 0, offset: 370 byte(1480+1480/8)

3. Reassembly

  • when fragmented datagrams arrive at final destination,
  • it is reassembled

💡 IP header for fragmentation, reassembly

  • for fragmentation and reassembly IP header has the following features
  • ID: all fragments from same datagram share a same ID
  • flag: to check until where this fragmented datagram ends
    • last flag of fragmented datagram is 0
    • all other flags are 1
  • fragmentation offset: order of fragmented datagram
    • to reassemble in order
    • byte

Screenshot 2024-09-11 at 18 17 26

📌 Subnet

subnet: isolated part of whole network
for subnet, detatch each inferface from host or router, and create island of isolated network

Screenshot 2024-09-11 at 18 23 55

  • three subnets(three blue parts)
  • for each subnet, first 24bits are identical 233.1.1
  • subnet mask: 24
    • means out of 32 bit address, first 24bits are subnet address
    • 💡 first 24bits: network addresss, last 8bits: host address
  • in each subnet, 2^8 - 2 number of IP address can exist

Why 2^8 - 2 number of IP address can exist?

host number 1111111 is used for broadcast address

  • broadcast address: 255.255.255.255
  • if host sends packet to broadcast address, all hosts in same subnet recieve packet
    host number 00000000 is used for network address 233.1.1.0


⭐️ Subnetting:

  • divide large network(class A, B, C) into smaller subnets
  • IP range 192.168.1.0/24 can be subnetted into 192.168.1.0/25, 192.168.1.128/25

📌 Subnet Mask

network bit:1, host bit: 0
32bit address
distinguish network address from host address

  • 255: broadcast address

  • 0: network address

  • 👍🏻 control number of hosts that can be on network depends on subnet mask
  • 👍🏻 make broadcasting domain smaller, send fewer broadcasts, lower network load
  • 👍🏻 safeguard network from another infiltration
  • 👎🏻 in order to communicate among different network, need router

⭐️ Subnet Masking:

  • use subnet mask to distinguish network part and host part of IP address

  • 255.255.255.0 (or /24) means that the first 24 bits of the IP address represent the network, and the remaining 8 bits represent the host

  • Default subnet masks
    • class C: 255.255.255.0
    • class B:255.255.0.0
    • class A: 255.0.0.0

Why do we use subnet masks?

  1. to make smaller parts of network
    if we use the IP address as recieved, broadcast domain is too big
    communication between subnets are possible through router
  2. save IP address


How can we communicate among different subnets?

with router

✅ IP address parts

IP address = network address ➕ host address

✔️ Network part

  • on same subnet
  • broadcast domain

  • can communicate without going through router

  • ⭐️ same for every device on same subnet

✔️ Host part

  • each PC/device
  • last octet
  • ⭐️ always different for each device

❓ Hub, switch, router?

  • on same network: hub, switch
    • look at host part of IP address
    • do not need router to communicate


  • on different subnet: router
    • look at different network part of IP address

What does it mean to have different network address?

  • to have different broadcast domain

✅ IP address class

Why do we divde address class?

to distribute IP address more efficiently

Screenshot 2024-09-11 at 18 46 49

✔️ Class A

network part 8bits(1byte) ➕ host part 24bits(3bytes)

  • start with 0
    • 0xxx xxxx. xxxx xxxx. xxxx xxxx. xxxx xxxx
  • maximum network address number: 0~2^7 (8-1)
  • maximum host address number: 2^24 - 2

✔️ Class B

network part 16bits(2bytes) ➕ host part 16bits(2bytes)

  • start with 10
    • 10xx xxxx. xxxx xxxx. xxxx xxxx. xxxx xxxx
  • maximum network address number: 0~2^14 (16-2)
  • maximum host address number: 2^16 - 2

✔️ Class C

network part 24bits(3bytes) ➕ host part 8bits(1byte)

  • start with 110
    • 110x xxxx. xxxx xxxx. xxxx xxxx. xxxx xxxx
  • maximum network address number: 0~2^21 (24-3)

  • maximum host address number: 2^8 - 2

  • 🛠️ normally used for public IP ddress


Why is class C used for public IP address?

as class A, B can have too many hosts

✔️ Class D

  • for multicast

✔️ Class E

  • for future uses

📌 CIDR

Classless Interdomain Routing

  • routing strategy
  • more flexible than classes
  • subnet portion of address of arbitary length
  • format: a.b.c.d/x
  • x: first number of bits, subnet(network) part of address

image

  • 👍🏻 more flexible divide IP address
  • 👍🏻 more efficiently use IPv$

📌 DHCP

Dynamic Host Configuration Protocol
protocol for host to get IP address
automated process of assigning IP address

Screenshot 2024-09-11 at 19 14 54

✔️ Two ways of getting IP address

  • device configure IP address manually

  • DHCP

  • server-client protocol
  • UDP
  • port 67, 68
  • If a new device connects to network, DHCP assigns IP address to the device
  • maintain unique IP address for each hsot

1. DHCP Discover message

  • new host connects to network
  • at this momemnt
    • the host does not know the IP address it will be connected to
    • thus, source IP address is 0.0.0.0(PC has had no IP address until now)
    • the host does not know DHCP adress
    • thus, destination IP address is 255.255.255.255(for broadcasting)
    • thus, sends broadcsting to send message to DHCP
  • sends DHCP discover message

2. DHCP offers a message

  • upon recieving message from host, DHCP sends message to host
  • server IP is specified in message packet to identify the server
  • source IP address is DHCP server IP address
  • destination IP address is 255.255.255.255(broadcast IP address)

3. DHCP request message

  • host recieves DHCP offer message
  • host produces ARP to check if there are any other host with same IP address
  • host sends DHCP request
  • source IP address is 0.0.0.0(PC still has no IP address)
  • destination IP address is 255.255.255.255(for broadcasting)

4. DHCP ACK message

  • upon ` DHCP request`, DHCP server sends DHCP ACK to check requested parameters
  • now, host has IP address provided by DHCP server

📌 Private IP, Public IP

✔️ Two types of IP address

  • public IP
  • private IP

✔️ Private IP

internal address of device that is not routed to internet

  • IP only for local network
  • private IP is non-routable on internet(not open to outside)
  • serach, access is impossible from outside

  • private IP is only used within private network

  • private IP can begin with

    • 192.168.xxx.xxx
    • 172.10.xxx.xxx
    • 10.xxx.xxx.xxx
  • 👍🏻 more security within particular network
  • 🛠️ used for internet wired/wireless connection

✔️ Public IP

  • has access to Internet

📌 NAT

Network Address Translation
translate public IP to private IP

  • use private IP in private network
  • use public IP to access Internet
  • NAT: allow multiple device to access Internet through single public address

Screenshot 2024-09-11 at 23 41 55

0. Environment

  • router IP: 138.76.29.7
  • PC private IP: 10.0.0.1

1. host send from private IP

  • host with private IP 10.0.0.1 requests web page to

  • web server with public IP 128.119.40.186(port 80)

  • host sends its private IP with port 3345

2. NAT recieves, translates

  • NAT translates private IP into public IP
  • create port 5001
  • translate to public IP 138.76.29.7

3. Web server replies

  • web server will reply to public IP 138.76.29.7
  • and port 5001

4. NAT translate

  • translate public IP to private IP 10.0.0.1

💡 Get network address, broadcast address

❓ My computer IP is 165.132.120.10
subnet mask is 255.255.252.0
What is my network address? and broadcast address?

  • 255.255.252.0 ➡️ 1111 1111.1111 1111.1111 1100.0 ➡️ 1이 22개
  • 따라서 165.132.120.10/22
  • 165.132.120.10 ➡️ 이진수로 나타낸 다음 ➡️ 앞에서 부터 22개
  • 8 + 8 + 6개니까

  • 120 ➡️ 01111000 ➡️ 011110 ➡️ 120

  • thus, network address is 165.132.120.0
  • broadcast address is 165.132.120.255

❓ My computer IP address is 165.132.120.10
subnet mask is default 255.255.0.0
What is my network address? and broadcast address?

  • network address: 165.132.0.0
  • broadcast address: 165.132.255.255

❓ My computer IP address is 165.132.120.10
subnet mask is default 255.255.255.0
What is my network address? and broadcast address?

  • network address: 165.132.120.0
  • broadcast address: 165.132.120.255

❓ My computer IP address is 192.168.51.111/20
Is my IP address private?

  • yes

❓ My computer IP address is 192.168.51.111/20
What is the network address? and broadcast address?

  • 51 ➡️ 00110011 ➡️ 20= 8 + 8 + 4니까 0011
  • network address: 192.168.48.0
  • broadcast address: 192.168.48.255

❓ My computer IP address is 192.168.51.111/20
Is 192.168.60.211 on the same network?

  • 60 ➡️ 00111100 ➡️ 앞에 4자리 0011
  • yes

❓ Can I assign IP address to 192.168.63.255

  • No. Cannot assign IP address to broadcasting address

❓ What is the network mask of 192.168.51.111/20?

  • first 20 digits of binary number in 1
  • 1111 1111.1111 1111.1111 0000.0000 0000
  • thus, 255.255.240.0

❓ Can 192.168.48.1 be the gateway of 192.168.51.111/20 network?

  • Yes.
  • all other addresses that are not broadcast or network can be gateway

🆚 Gateway

✔️ router

  • route data packet in similar networks
  • OSI layer 3, 4
  • NAT, DHCP

✔️ gateway

  • connect two network, different protocols as a translator
  • connects two dissimilar networks
  • OSI layer 5
  • network access control, protocol conversion

🆚 Mac address

✔️ Mac address

  1. Physical Identifier
  • physical identifier: identifier for NIC card on local network
  • identify physical device itself on local network
  1. Layer 2
  • datalink layer
  • switch, bridge
  • make sure data is sent to correct physical device on local network
  • smame network
  1. Format
  • 48-bit number
  • example: 00:1A:2B:3C:4D:5E or 00-1A-2B-3C-4D-5E

✔️ IP address

  1. Logical Identifier
  • logical identifier: identifier of device connected to network(Internet)
  • device location on network that uses Internet Protocol
  1. Layer 3
  • network layer
  • router
  • different network
  1. Format
  • 32-bit number(IPv4) or 128-bit number(IPv6)
  • example: 192.168.1.1, 2001:0db8:85a3:0000:0000:8a2e:0370:7334

✔️ Address Resolution Protocol

  • device use ARP to map IP address to MAC address
Computer Science, Network
This post is licensed under CC BY 4.0 by the author.