Книга: Acing the CCNA Exam: Volume 1 Fundamentals and Protocols
Назад: Part 1. Network fundamentals
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7 . Try it a few times each day; as you practice and improve, your scores in the Binary Game should increase, and you’ll find yourself able to do the necessary calculations in your head..

  • An IPv4 address is a 32-bit number that identifies a host at Layer 3. It is divided into four groups of 8 bits called octets and written in dotted decimal notation.

  • IPv4 addresses are divided into two parts: the network portion and the host portion. All hosts within a LAN will have the same network portion but a unique host portion.

  • The size of the network portion can be indicated with a prefix length in the format /X, where X is the number of bits in the network portion. Any bits that are not part of the network portion are part of the host portion.

  • The size of the network portion can also be indicated with a netmask (also called a subnet mask). A netmask is a string of 32 bits that is paired with an IP address to indicate which bits of the IP address are the network portion and which are the host portion.

  • A 1 in the netmask means the bit in the same position as the IP address is part of the network portion. A 0 in the netmask means the bit in the same position as the IP address is part of the host portion.

  • The show ip interface brief command lists a router’s interface and information about their IP addresses and status.

  • The show ip interface [interface-name] command shows more detail about each interface.

  • Router interfaces are disabled by default and must be enabled with the no shutdown command.

  • Interface configuration mode can be accessed with the interface interface-name command from global configuration mode.

  • An interface’s IPv4 address can be configured with the ip address ip-address netmask command in interface configuration mode.

  • The network address of a network is the first address of the network, with a host portion of all 0s. It is used to identify the network and cannot be assigned to a host.

  • The broadcast address of a network is the last address of the network, with a host portion of all 1s. It can be used to send a message to all hosts in the network. However, to send a message to all hosts on the local network, the address 255.255.255.255 is usually used.

  • The maximum number of hosts of a network is the number of IP addresses that can be assigned to hosts. The formula is 2y − 2, where y is the number of bits in the host portion. Two is subtracted for the network and broadcast addresses.

  • The first usable address of a network is the first address that can be assigned to a host. The last usable address is the last address that can be assigned to a host.

  • IPv4 addresses can be organized into five classes: A, B, C, D, and E. Class D is reserved for multicast addresses, and Class E is reserved for experimental purposes. Addresses from classes A, B, and C are assigned to network hosts.

  • Class A addresses have a first octet of 0–127 and use a /8 prefix length. Class B addresses have a first octet of 128–191 and use a /16 prefix length. Class C addresses have a first octet of 192–223 and use a /24 prefix length.

  • Networks that follow class A, B, and C rules are called classful networks. This system is now obsolete and has been replaced with classless networking, which is more flexible.

    • Назад: Part 1. Network fundamentals
    Дальше: Part 2. Routing fundamentals and subnetting