GATE CS 2014 Set 3 Q27 — Fragmentation and IP Addressing, IPv4, CIDR

To design a $50\text{-bit}$ globally unique ID, we divide the bits into global and local components:

• Global Identifier: Since each host has a globally unique $\textsf{IPv4}$ address, we use the $32\text{-bit}$ $\textsf{IPv4}$ address to ensure global uniqueness.
• Local Identifier: This leaves $50 - 32 = 18$ bits for generating unique IDs locally on each host.

The host has a $1\text{-second}$ resolution real-time clock and needs to generate up to $1000$ unique IDs per second. To achieve this, the $18\text{-bit}$ local identifier is split into two parts:

1. Counter: To distinguish up to $1000$ IDs generated within the same second, we need a counter. The number of bits required is:$$\lceil \log_2(1000) \rceil = 10 \text{ bits}$$
2. Timestamp: The remaining bits are allocated to the timestamp (which increments every second):$$18 - 10 = 8 \text{ bits}$$

With $8$ bits allocated for the timestamp, the maximum number of unique seconds that can be represented before wrapping around is: