IP Addressing Concepts | CompTIA Network+ N10-007 | 1.4a

In this video you will learn about IP addressing concepts such as: private vs. public IP addressing, reserved IP addresses, loopback addresses, default gateway, virtual IP address, DHCP, DHCPv6, static IP addresses, APIPA, & IP reservation.

Private vs. Public IP Addressing

A public IP address (external IP address) is assigned to every device that connects to the Internet and each IP address is unique.  This addressing scheme makes it possible for the devices to find each other online and exchange information.  A user has no control over the public IP address that is assigned to the device.  The public IP address is assigned to the device by the ISP as soon as the device is connected to the Internet.  Also, a public IP address can be static, dynamic, or even shared.

In IP networking, a private network is a computer network that uses private IP address space.  These addresses are commonly used for local area networks (LANs) in residential, office, & enterprise environments to allow for devices to communicate with each other on the same network.  Private network addresses are not allocated to any specific organization and anyone may use these addresses without approval from regional or local Internet registries.  

An IP address is considered private if the IP number falls within one of the IP address ranges reserved for private networks such as a LAN.  The IANA (Internet Assigned Numbers Authority) has reserved the following three blocks of the IP address space for private networks (local networks).  In order to communicate with the outside world, these IP addresses must be translated to a public IP address using NAT (network address translation):

  • – (total addresses:  16,777,216)
  • – (total addresses:  1,048,576)
  • – (total addresses:  65,536)
Public vs. Private IP Address

Reserved IP Addresses

In the Internet addressing architecture, the Internet Engineering Task Force (IETF) and the Internet Assigned Numbers Authority (IANA) have reserved various IP addresses for special purposes.  Types of reserved IP addresses are:

  • Private IP addresses
  • Loopback addresses
  • Addresses in the range from – (these IP address don’t do anything & will not function properly no matter where on the network they are configured)

Loopback IP Addresses

A loopback IP address is a special IP address reserved for use in testing network cards.  This IP address corresponds to the software loopback interface of the network card, which does not have hardware associated with it, and does not require a physical connection to a network.  The loopback address allows for a reliable method of testing the functionality of an Ethernet card, its drivers, and software without a physical network.  It also allows IT professionals to test IP software without worrying about broken or corrupted drivers or hardware.

  • IPv4 loopback address: (technically everything from –
  • IPv6 loopback address:  0:0:0:0:0:0:0:1 or ::1
Loopback IP Address

Default Gateway

A default gateway is a network node that serves as the forwarding host (router) to other networks, often involving not only a change of addressing, but also a different networking technology.  More narrowly defined, a router merely forwards packets between networks with different network prefixes.  The networking software stack of each computer contains a routing table that specifies which interface is used for transmission and which router on the network is responsible for forwarding to a specific set of addresses.  If none of these forwarding rules is appropriate for a given destination address, the default gateway is chosen as the router or last resort.  In a small office home office (SOHO) environment, the default gateway is a device, such as a DSL router or cable router, that connects the local network to the Internet.  It serves as the default gateway for all network devices.  Enterprise networks may require internal network segments where a device wishing to communicate with a host on the public Internet would have to forward the packets to the default gateway for its network segment which would have a default route configured to a device on an adjacent network which would be one hop closer to the public Internet.

Default Gateway

Virtual IP Address

A virtual IP address is an IP address assigned to multiple domain names or servers that share an IP address based on a single network interface card (NIC).  Virtual IPs are allocated to virtual private servers, websites or any other application residing on a single server.  The host server for these applications has a network IP address assigned by a network administrator, whereas the different server applications have virtual IPs.  In layman’s terms, a server IP address depends on the Media Access Control (MAC) address of the attached NIC, and only 1 logical IP address may be assigned per card.  However, virtual IP addressing enables hosting for several different applications and virtual appliances on a server with only one logical IP address.  The virtual IP is used for communication by all hosted applications, even though data packets are routed through actual network interfaces.  Virtual IPs are primarily implemented for the following reasons:

  • To consolidate resources through the allocation of one network interface per hosted application.
  • To improve redundancy by providing alternative failover options on one machine.
Virtual IP Address

Address Assignments


Dynamic Host Configuration Protocol is a network management protocol used on IP networks whereby a DHCP server dynamically assigns an IP address and other network configuration parameters to each device (PCs, printers, servers, routers, etc.) on a network so they can communicate with other IP networks.  A DHCP server enables computers to request IP addresses and networking parameters automatically from the ISP, reducing the need for a network administrator or a user to manually assign IP addresses to all network devices.  In the absence of a DHCP server, a computer or other device on the network needs to be manually assigned an IP address, or to assign itself an APIPA address, the latter of which will not enable it to communicate outside its local network.  If you get your address from a DHCP server, you are getting your address assigned dynamically, and it could change periodically.  Some devices on the network require static addresses that do not change, such as printers & servers which have to be assigned manually by a network administrator.  This enables these devices to be more reliable and easily accessible over time.  DHCP uses ports 67 & 68, where UDP port number 67 is the destination port of a server, and UDP port number 68 is used by the client.



DHCPv6 is a network protocol for configuring IPv6 hosts with IP addresses, IP prefixes & other configuration data required to operate in an IPv6 network.  It is the IPv6 equivalent of the DHCP for IPv4.  DHCPv6 uses UDP port number 546 for clients and port number 547 for servers.

Static IP Address

A static IP address is simply an IP address that doesn’t change.  Once your device is assigned a static IP address, that number typically stays the same until the device is decommissioned or your network architecture changes.  Static IP addresses generally are used by servers or other important equipment that devices need to have constant access to.

Static IP Address


If a router does not have enough IP addresses for the devices that need to connect to it, APIPA IP addresses (from – are assigned to devices arriving after the pool of addresses are used up. APIPA stands for Automatic Private IP Addressing which is a feature in Windows-based operating systems that enables computers to automatically self-configure an IP address and subnet mask when their DHCP server isn’t reachable. The APIPA service checks regularly for the presence of the DHCP server every 3 minutes. If it detects a DHCP server on the network, the DHCP server will replace the APIPA networking address with dynamically assigned addresses.


IP Reservation

DHCP IP reservation allows for the same IP address to be assigned to a specific device whenever that device connects to the network.  Think of it like a reserved parking spot at an apartment complex.  Most devices use DHCP, which assigns dynamic IP addresses as a default.  But sometimes, you may want or need devices to always have the same IP address.  For example, a wireless printer is a device that should have the same IP address at all times.  When you print something, your computer & printer locate each other using IP addresses.  If your printer’s IP address keeps changing, your computer may not always be able to find the printer.  Giving your printer a static IP address will ensure that your computer always knows the address of your printer as it won’t change if it’s rebooted or if a DHCP IP lease expires.

IP Reservation