Ravindra BagaleCourses & study guides

1. IP Addressing: IPv4, Subnets, Public and Private IPs

1.6 Static vs Dynamic IP and DHCP

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Ghari Wi-Fi la connect kelyavar tumhi IP type karta ka? Nahi na? To DHCP deto. Server la matra fix address lagto. Chala baghu farak.

Feature Static IP Dynamic IP
Assigned by Manually by an admin (or reserved) Automatically by a DHCP server
Changes? Never, unless changed manually May change on reboot / lease expiry
Best for Servers, DNS records, printers, VPN endpoints Laptops, phones, home users
Cost Often extra (ISPs charge) Usually included
AWS equivalent Elastic IP Auto-assigned public IPv4

DHCP (Dynamic Host Configuration Protocol) hands out IP address, subnet mask, default gateway and DNS server in four steps: Discover → Offer → Request → Acknowledge (DORA).

  Client (no IP yet)                         DHCP server (router)
        |  1. DISCOVER  (broadcast: "any DHCP server?")   |
        | ------------------------------------------------> |
        |  2. OFFER     ("take 192.168.1.23")               |
        | <------------------------------------------------ |
        |  3. REQUEST   ("I want 192.168.1.23")             |
        | ------------------------------------------------> |
        |  4. ACK       ("done, lease 24 h, gw .1, dns .1") |
        | <------------------------------------------------ |

Why this matters for security

Because DHCP trusts the first answer, an attacker on the same LAN can run a rogue DHCP server and hand out their own machine as the gateway or DNS server – a classic man-in-the-middle setup. Switches defend with DHCP snooping. On the server side, anything that must be reachable or referenced in DNS needs a static address (on AWS, an Elastic IP).

Ravindra Bagale's Tip

Many students put an EC2 instance's auto-assigned public IP in DNS, stop the instance at night, and in the morning the website is down! The public IP changes after a stop/start. Give an Elastic IP to anything that needs a fixed address – we'll do this hands-on in Part 6. Remember this.

Practice task

On Windows run ipconfig /all and find DHCP Enabled, Lease Obtained and DHCP Server. On Linux run ip route and note your default gateway.