Bangladesh Television Assistant Programmer Exam: 2019প্রশ্ন ও সমাধান

Private IP address: an IP address reserved for use only within a private/internal network (LAN) — it is never routed on the public internet. Devices with private IPs reach the internet only via Network Address Translation (NAT) on a router/gateway, which maps the private address to a public one.

Private IP ranges (reserved by IANA, RFC 1918):

Class	Range	Approx. Total Addresses
Class A	10.0.0.0 - 10.255.255.255	~16.7 million
Class B	172.16.0.0 - 172.31.255.255	~1 million
Class C	192.168.0.0 - 192.168.255.255	~65,536

Class B private IP range: 172.16.0.0 to 172.31.255.255

Why private IPs exist: the public IPv4 address space (about 4.3 billion addresses) is far too small for every device worldwide to have a unique public address — private ranges let organizations reuse the same internal addresses freely (every home router's LAN can use 192.168.1.x independently), while NAT hides this reuse from the internet.

Example: an office with 500 employees might assign each workstation an address like 172.16.5.23 (Class B private range, chosen for its larger address pool than Class C) — none of these addresses are reachable directly from the internet; only the router's single public IP is.

Exam takeaway: private IP ranges are defined per class (10.x for A, 172.16-172.31.x for B, 192.168.x for C) — memorizing the 172.16-172.31 boundary specifically is the key exam detail for Class B.

Access Time: the total time taken between a request for data and the moment that data is actually delivered/ready.

• Memory (RAM) access time: how long it takes to read/write a location in RAM — typically a few nanoseconds (ns) for modern DRAM.
• Disk access time: how long it takes to reach and read the first byte of requested data — composed of seek time (moving the read/write head to the correct track) plus rotational latency (waiting for the correct sector to rotate under the head). Because these vary with the disk's current head/platter position, disk access time is always quoted as an average.

Transfer Time: once access has occurred, transfer time is how long it takes to actually move the requested data:

Transfer Time = Data Size / Transfer Rate

Worked example: transferring a 100 MB file over a link with a transfer rate of 20 MB/s:
Transfer Time = 100 / 20 = 5 seconds

Why the distinction matters: total time to retrieve data = Access Time + Transfer Time. For small, scattered reads (like a database doing many small lookups), access time dominates (SSDs help enormously here, since they have near-zero seek time); for large sequential reads (like copying a big video file), transfer time dominates, so raw bandwidth matters more.

Exam takeaway: access time = time to locate and begin reading data (seek plus latency for disks); transfer time = time to actually move the data once located — an SSD's advantage over an HDD is primarily in access time, not necessarily raw transfer rate.