How many IP assignable addresses are contained in 10.0.128.0/23?

The CIDR notation /23 indicates that 23 bits are used for the network portion of the address. An IPv4 address has a total of 32 bits, leaving 9 bits for the host portion (32 - 23 = 9). The total number of addresses in this subnet is 2^9, which equals 512. However, the question asks for "assignable" addresses. In any subnet, two addresses are reserved: the network address (all host bits are 0) and the broadcast address (all host bits are 1). Therefore, the number of assignable addresses is 512 - 2 = 510.

A. 4094 assignable addresses corresponds to a /20 network (2^12 - 2), which has 12 host bits, not 9.

B. 254 assignable addresses corresponds to a /24 network (2^8 - 2), which has 8 host bits, not 9.

D. 512 is the total number of addresses in the /23 subnet, but it includes the non-assignable network and broadcast addresses.

1. Comer, D. E. (2018). Internetworking with TCP/IP Volume 1: Principles, Protocols, and Architecture (6th ed.). Pearson. Chapter 10, Section 10.9 "A Classless Addressing Example", discusses calculating the number of hosts from the prefix length.

2. Fall, K. R., & Stevens, W. R. (2011). TCP/IP Illustrated, Volume 1: The Protocols (2nd ed.). Addison-Wesley Professional. Chapter 2, Section 2.2 "IP Addresses", explains the structure of IP addresses and subnets.

3. Fuller, V., & Li, T. (2006). Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan. RFC 4632. Section 3.1 "CIDR Properties" explains that a prefix of length /N corresponds to 2^(32-N) individual host addresses.