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IP Addressing

  • Ops
tags: ccna

IP Addressing

  • layer 3 logical address assigned by an administrator(MAC built in NIC)
  • used to identify specific devices on a network
  • every device on the internet has a unique IP Address

Street Analogy

  • network address portion
    • identifies a specific network
    • routers route traffic via routing tables, is based on network address(Network ID), not ip address
  • host address portion
    • identifies a specific endpoint on a network
    • we can use a protocal such as ARP to find the host


  • connectionless protocal: no sessions formd when transmitted, no status info
  • packets treated independently
    • may take different paths: load balancing, bandwidth, hopcount
  • hierarchical addressing sturture
  • best effort delivery
  • format
    • 32 bit with 4 octets
    • like DHL or FedEx routing parcel based on an address


  • Unicast Traffic
    • A
      • start with binary 0
      • range from 0(00000000).0.0.0 to 127(01111111).255.255.255
      • exceptions:
        • 127 is reserved for loopback:
        • 0 network is reserved for default network:
      • actual range from to
      • portions
        • first 1 octets: Networks
        • last 3 octests: Hosts
    • B
      • start with binary 10
      • range from 128(10000000).0.0.0 to 191(10111111).255.255.255
      • portions
        • first 2 octets: Networks
        • last 2 octests: Hosts
    • C
      • start with binary 110
      • range from 192(11000000).0.0.0 to 223(11011111).255.255.255
      • portions
        • first 3 octets: Networks
        • last 1 octests: Hosts
  • Multicast
    • D
      • start with binary 111
  • reserved for other purposes
    • E
      • start with binary 1111

These classes replaced by Classless Inter-Domain Routing(CIDR) in 1993

Special Address

  • Directed Broadcast Address
    • host sends data to all device on a specific network
    • binarys 1s in the entire host portion
    • routers can route directed broadcast, disabled by default
    • Denial of service attack(DDoS)
  • Local Broadcast Address
    • communicate with all device on local network
    • address is all binary 1 (
    • e.g. host requrest an IP address from an DHCP Server(Dynamic Host Configuration Protocal)
    • always dropped by routers, configuring DHCP forwarding or DHCP relay
  • Local Loopback Address
    • let a system send a message to itself for testing
    • to make sure that the TCP/IP stack is correctly installed
    • typical use with IPv4
      • class A
      • anything in 127 range is deemd as loopback address: 16 million ip wasted
    • also have IPv6 loopback suchas ::1, resolve the IPv4 wasted.
    • routers and switchers can also be configured with a IP address on a loopback interface, which are not the same as local loopback address

Private Address

See RFC1918

  • 1 class A networks
  • 16 class B networks
  • 256 class C networks
  • Microsoft IPv4 Link Local Address

See RFC3927

  • Automactic Private IP Address (APIPA)
  • PC automatically chooses an IP address in range with no DHCP
  • can immediately communicate using IP without configuration: only on LOCAL LINK
  • non routable

Subnet Masks

  • to determin the portion of address which is host and network
    • Remote: different sebnet, reached via a default gateway
    • Local: same subnet, communicate on local segment directly, does not require a default gateway

Class A, B and C have default masks:

  • Class A:
  • Class B:
  • Class C:

Two simple rules, any address bits have corresponding mask bit:

  • set to 1: represent the NETWORK ID
  • set to 0: represent the HOST ID

To work out if another host is local or remote

  • check network portion
  • compare to the other host

Subnet mask need to be CONTIGUOUS:

CIDR Notation

  • Replace classful IP Addressing
  • Variable Length Subnet Mask(VLSM)
    • Use notation rather than
  • Vary Mask and allocate ip ranges dynamically instead of classful masks


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