Default gateway

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

A default gateway is the router or forwarding path used to send traffic toward destinations outside a device's known networks.

A default gateway is the router a host uses to send packets outside its local network. Hosts on the same network communicate directly without using the gateway. section 9.1 section 9.4

When a host sends traffic to a remote destination:

• The host compares the destination IP address with its own network and determines that the destination is outside the local network. section 10.1.1
• It keeps the remote destination in the packet but addresses the local Ethernet frame to the gateway interface's MAC address. section 9.1
• ARP discovers the gateway's MAC address; the gateway then examines the packet, selects a routing-table entry, and forwards it in a new frame. section 9.2 section 10.3

PC1 sends to PC3

PC1 at 192.168.1.11 sends to PC3 at 192.168.2.11. Because the networks differ, PC1 sends a frame to R1's G0/0 MAC address. R1 routes the packet and sends a new frame toward PC3's MAC address. A common gateway address is 192.168.1.1, although the gateway need not use the first usable address. section 9.1

Figure 10.2 PC1 uses ARP to learn R1 G0/1’s MAC address. (1) PC1 sends an ARP request to 192.168.1.1. SW1 learns PC1’s MAC address and floods the frame due to the destination MAC address of ffff.ffff.ffff. (2) After receiving the ARP request, R1 adds an ARP table entry associating IP address 192.168.1.11 with PC1’s MAC address. R1 then sends an ARP reply to PC1. SW1 learns R1 G0/1’s MAC address and forwards the frame to PC1. (3) After receiving the ARP reply, PC1 adds an ARP table entry associating IP address 192.168.1.1 with R1 G0/1’s MAC address.

Follow the ARP exchange in which PC1 requests 192.168.1.1, R1 replies with its interface MAC address, and both devices update their ARP tables. section 10.1.1

Administrators can configure a gateway manually, but user devices commonly learn it through DHCP. On Windows, ipconfig displays the IPv4 address, subnet mask, and default gateway. Do not confuse a host's default gateway with a router's default route: a router's 0.0.0.0/0 route forwards destinations that match no more-specific route. section 9.1 section 9.3.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Traces subnet comparison, ARP resolution, frame addressing, router lookup, and forwarding, while deferring detailed routing-table construction.

Examples

Uses PC1, PC2, PC3, R1, ipconfig output, ARP exchange, and Cisco routing-table output.

What it assumes

Assumes basic IP addressing and subnet-mask use while building router, ARP, frame, and routing-table concepts.

What you can do after

Diagnose gateway addressing and ARP behavior using ipconfig and packet-flow reasoning.

How it connects

Connects default gateways with routers, routing tables, routing, ARP, DHCP, subnet masks, MAC addresses, and default routes.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 1 Fundamentals and Protocols on liveBook Get the book at manning.com

Also explained in

• 

  liveBookmanning.com
  Acing the CCNA Exam: Volume 2 Advanced Networking and SecurityA default gateway is the router to which a host sends packets destined for networks absent from its routing table.

Compare how 2 books explain default gateway →

Related concepts

• routerkind of · A router is a network device that forwards packets between networks based on destination addresses and routing information.
• dhcpproduced by · The Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns devices IP addresses and related configuration information.
• first hop redundancy protocolproduced by · A first-hop redundancy protocol provides hosts with resilient default-gateway service through a virtual gateway backed by multiple routers.
• arpused by · The Address Resolution Protocol is a local-network protocol that maps an IPv4 address to the corresponding link-layer address, typically a MAC address.
• virtual ip addressconfigured by · A virtual IP address is a logical network address representing a service or routing endpoint independently of a particular physical host or interface.
• network switchused by · A network switch is a device that connects devices within a local network and selectively forwards data-link-layer frames to their intended destinations.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.