Untitled Flashcard Set

Prefix Length
- Represents the number of contiguous bits set to binary $1$ in the subnet mask.
- Expressed in Classless Inter-Domain Routing (CIDR) slash notation (e.g., $/24$ corresponds to a subnet mask of $255.255.255.0$ ).
- Defines the boundary separating the network portion from the host portion of an IP address.
Logical AND Operation
- A bitwise Boolean operation performed by networking devices (such as hosts and routers) to determine the destination network address.
- Truth Table Rules:
- $1 \text{ AND } 1 = 1$
- $1 \text{ AND } 0 = 0$
- $0 \text{ AND } 1 = 0$
- $0 \text{ AND } 0 = 0$
- An IPv4 address is compared bit-by-bit against the subnet mask. Where mask bits are $1$ , the IP bit is preserved; where mask bits are $0$ , the result is always $0$ .
Octet Boundary
- Occurs when subnetting strictly aligns with $8$ -bit boundaries ( $/8$ , $/16$ , $/24$ , and $/32$ ).
- At an octet boundary, subnet calculations do not require binary conversion because entire octets are dedicated entirely to the network or host portions.

Network Address
- The very first address in a subnet, where all host bits are set to binary $0$ .
- Serves as the unique identifier for the entire subnet.
- Cannot be assigned to an individual host interface.
Broadcast Address
- The very last address in a subnet, where all host bits are set to binary $1$ .
- Used to deliver packets simultaneously to every host on that specific local subnet.
- Cannot be assigned to an individual host interface.
First Usable Address
- The numerical address immediately following the network address.
- Calculated as: $\text{Network Address} + 1$ .
Last Usable Address
- The numerical address immediately preceding the directed broadcast address.
- Calculated as: $\text{Broadcast Address} - 1$ .
Usable Host Formula
- For a subnet with $n$ host bits, total usable host addresses are calculated by: $\text{Usable Hosts} = 2^n - 2$
- The subtraction of $2$ accounts for the reserved network and broadcast addresses.

Public vs. Private Addresses
- Public Addresses: Globally unique IPv4 addresses routable across the global Internet, assigned through authorized registries.
- Private Addresses (RFC 1918): Reserved for internal local area networks; non-routable on the public Internet.
- Class A Range: $10.0.0.0/8$ ( $10.0.0.0$ to $10.255.255.255$ )
- Class B Range: $172.16.0.0/12$ ( $172.16.0.0$ to $172.31.255.255$ )
- Class C Range: $192.168.0.0/16$ ( $192.168.0.0$ to $192.168.255.255$ )
Network Address Translation (NAT)
- A method enabling multiple devices on a private network to share one or a small pool of public IP addresses.
- Modifies IP headers in transit, mapping internal private IP:port pairs to external public IP:port pairs, conserving public IPv4 space.
Loopback Addresses
- Block: $127.0.0.0/8$ (most commonly $127.0.0.1$ / localhost).
- Packets sent to this address loop directly back within the local operating system without being transmitted across any physical network interface.
- Used for troubleshooting the local TCP/IP protocol stack and running local server processes.
Automatic Private IP Addressing (APIPA)
- Block: $169.254.0.0/16$ ( $169.254.0.1$ to $169.254.255.254$ ).
- Automatically self-assigned by an operating system when a DHCP client fails to discover or reach a DHCP server.
- Allows communication strictly with other devices on the same local Layer 2 broadcast link, but cannot cross routers.

Classful Addressing
- Historical system allocating static blocks based on leading bit patterns:
- Class A: Range $0.0.0.0$ to $127.255.255.255$ (Default mask: $/8$ ; Leading bit: 0)
- Class B: Range $128.0.0.0$ to $191.255.255.255$ (Default mask: $/16$ ; Leading bits: 10)
- Class C: Range $192.0.0.0$ to $223.255.255.255$ (Default mask: $/24$ ; Leading bits: 110)
- Class D: Range $224.0.0.0$ to $239.255.255.255$ (Reserved for Multicast; Leading bits: 1110)
- Class E: Range $240.0.0.0$ to $255.255.255.255$ (Reserved for Experimental/Research purposes; Leading bits: 1111)
IANA and Regional Internet Registries (RIRs)
- Internet Assigned Numbers Authority (IANA): The central authority responsible for global IP address allocation and root zone DNS management.
- Regional Internet Registries (RIRs): Five regional entities that receive large IP blocks from IANA and distribute them to ISPs and local enterprises:
- ARIN: North America
- RIPE NCC: Europe, Middle East, parts of Central Asia
- APNIC: East Asia and Pacific region
- LACNIC: Latin America and the Caribbean
- AFRINIC: Africa

Broadcast Domains
- A logical division of a computer network in which any device can reach all other devices via a data link broadcast.
- Layer 2 switches forward broadcasts to all ports within a VLAN.
- Layer 3 routers define the boundary of broadcast domains and do not forward Layer 2 broadcasts by default.
Subnets
- Subdividing a larger network into multiple smaller, logically isolated segments by borrowing bits from the host portion to enlarge the network portion.
- Improves security, minimizes broadcast traffic congestion, and simplifies administrative organization.
Variable-Length Subnet Mask (VLSM)
- The practice of applying different subnet mask lengths to individual subnets within the same parent network.
- Also known as "subnetting a subnet."
- Solves the inefficiency of fixed-length subnetting by tailoring the subnet size to match the exact host requirements of each segment (e.g., using $/30$ or $/31$ for point-to-point router links to eliminate wasted host addresses).