What OSPF command, when configured, will include all interfaces into area 0?
A.
network 0.0.0.0 255.255.255.255 area 0
B.
network 0.0.0.0 0.0.0.0 area 0
C.
network 255.255.255.255 0.0.0.0 area 0
D.
network all-interfaces area 0
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Default Routes
Default routes, also known as gateways of last resort, are used to route packets that are addressed to networks not explicitly listed in the routing table. A device might not be able to determine routes to all networks. To provide complete routing capability, network administrators use some devices as smart devices and give the remaining devices default routes to the smart device. (Smart devices have routing table information for the entire internetwork.) Default routes can be either passed along dynamically or configured manually into individual devices.
Most dynamic interior routing protocols include a mechanism for causing a smart device to generate dynamic default information, which is then passed along to other devices.
You can configure a default route by using the following commands:
ip default-gateway
ip default-network
ip route 0.0.0.0 0.0.0.0
You can use the ip default-gateway global configuration command to define a default gateway when IP routing is disabled on a device. For instance, if a device is a host, you can use this command to define a default gateway for the device. You can also use this command to transfer a Cisco software image to a device when the device is in boot mode. In boot mode, IP routing is not enabled on the device.
Unlike the ip default-gateway command, the ip default-network command can be used when IP routing is enabled on a device. When you specify a network by using the ip default-network command, the device considers routes to that network for installation as the gateway of last resort on the device.
Gateways of last resort configured by using the ip default-network command are propagated differently depending on which routing protocol is propagating the default route. For Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP) to propagate the default route, the network specified by the ip default-network command must be known to IGRP or EIGRP. The network must be an IGRP- or EIGRP-derived network in the routing table, or the static route used to generate the route to the network must be redistributed into IGRP or EIGRP or advertised into these protocols by using the network command. The Routing Information Protocol (RIP) advertises a route to network 0.0.0.0 if a gateway of last resort is configured by using the ip default-network command. The network specified in the ip default-network command need not be explicitly advertised under RIP.
Creating a static route to network 0.0.0.0 0.0.0.0 by using the ip route 0.0.0.0 0.0.0.0 command is another way to set the gateway of last resort on a device. As with the ip default-network command, using the static route to 0.0.0.0 is not dependent on any routing protocols. However, IP routing must be enabled on the device. IGRP does not recognize a route to network 0.0.0.0. Therefore, it cannot propagate default routes created by using the ip route 0.0.0.0 0.0.0.0 command. Use the ip default-network command to have IGRP propagate a default route.
EIGRP propagates a route to network 0.0.0.0, but the static route must be redistributed into the routing protocol.
Depending on your release of the Cisco software, the default route created by using the ip route 0.0.0.0 0.0.0.0 command is automatically advertised by RIP devices. In some releases, RIP does not advertise the default route if the route is not learned via RIP. You might have to redistribute the route into RIP by using the redistribute command.
Default routes created using the ip route 0.0.0.0 0.0.0.0 command are not propagated by Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS). Additionally, these default routes cannot be redistributed into OSPF or IS-IS by using the redistribute command. Use the default-information originate command to generate a default route into an OSPF or IS-IS routing domain.
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255.255.255.0 is the most common subnet mask used on computers connected to Internet Protocol (IPv4) networks. Besides its use on home network routers, a person will also commonly encounter this mask on network professional certification exams like the CCNA.
255.255.255.0 and Subnetting
Traditional subnets worked with so-called classful networks that partitioned the IP addresses into one of five classes (Class A/B/C/D/E) according to the value of the IP address number.
The subnet mask 255.255.255.0 converts to a 32-bit binary value
11111111 11111111 11111111 00000000
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https://learningnetwork.cisco.com/thread/7814
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By convention, area 0 (zero), or 0.0.0.0, represents the core or backbone area of an OSPF network. The identifications of other areas may be chosen at will; often, administrators select the IP address of a main router in an area as area identification. Each additional area must have a direct or virtual connection to the OSPF backbone area. Such connections are maintained by an interconnecting router, known as area border router (ABR).
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