A dual-homed host uses two NICs to attach to two separate networks,
commonly a trusted network and an untrusted network. It's
important that the internal routing function of the host be disabled to create an application-layer chokepoint and filter packets. Many systems come with routing enabled by default, such as IP forwarding, which makes the firewall useless. The other answers are distracters. Source:
Hacker Proof by Lars Klander (Jamsa Press, 1997).

Each Class C network address has a 24-bit network prefix, with the three highest
order bits set to 1-1-0
The following answers are incorrect:
The first bit of the IP address would be set to zero. Is incorrect because, this would be a Class A
network address.
The first bit of the IP address would be set to one and the second bit set to zero. Is incorrect
because, this would be a Class B network address .
The first three bits of the IP address would be set to one. Is incorrect because, this is a distractor.
Class D & E have the first three bits set to 1. Class D the 4th bit is 0 and for Class E the 4th bit to
1.Classless Internet Domain Routing (CIDR)
High Order bits are shown in bold below.
For Class A, the addresses are 0.0.0.0 - 127.255.255.255
The lowest Class A address is represented in binary as 00000000.00000000.0000000.00000000
For Class B networks, the addresses are 128.0.0.0 - 191.255.255.255.
The lowest Class B address is represented in binary as 10000000.00000000.00000000.00000000
For Class C, the addresses are 192.0.0.0 - 223.255.255.255
The lowest Class C address is represented in binary as 11000000.00000000.00000000.00000000
For Class D, the addresses are 224.0.0.0 - 239.255.255.255 (Multicast)
The lowest Class D address is represented in binary as 11100000.00000000.00000000.00000000
For Class E, the addresses are 240.0.0.0 - 255.255.255.255 (Reserved for future usage)
The lowest Class E address is represented in binary as 11110000.00000000.00000000.00000000
Classful IP Address Format
References:
3Com http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf
AIOv3 Telecommunications and Networking Security (page 438)