3Com 3 manual Subnet Mask Numbering, Extending the Network Prefix

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Subnets and Subnet Masks

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As shown in this example, the 32 bits of an IP address and subnet mask are usually written using an integer shorthand. This notation translates four consecutive 8-bit groups (octets) into four integers that range from 0 through 255. The subnet mask in the example is written as 255.255.255.0.

Traditionally, subnet masks were applied to octets in their entirety. However, one octet in the subnet mask can be further subdivided so that part of the octet indicates an extension of the network number, and the rest of the same octet indicates the host number, as shown in Figure 29.

Figure 29 Extending the Network Prefix

IP address

	Take the IP address
Network	Subnet and Host

Apply the subnet mask

Subnet mask

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0
Result = subnet/host boundary
Networknetwor	Subnetsubn	Host

Using the Class B IP address from Figure 28 (158.101.230.52), the subnet mask is 255.255.255.240.

The number that includes both the Class B natural network mask (255.255) and the subnet mask (255.240) is sometimes called the extended network prefix.

Continuing with the previous example, the subnetwork part of the mask uses 12 bits, and the host part uses the remaining 4 bits. Because the octets are actually binary numbers, the number of subnetworks that are possible with this mask is 4,096 (212), and the number of hosts that are possible in each subnetwork is 16 (24).

Subnet Mask Numbering

An alternate method to represent the subnet mask numbers is based on the number of bits that signify the network portion of the mask. Many Internet Service Providers (ISPs) now use this notation to denote the subnet mask. See Table 9.

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Contents SuperStack 3Com Corporation 5400 Bayfront Plaza Santa Clara, California Contents Using Multicast Filtering Status Monitoring and Statistics IP Addressing About this Guide List conventions that are used throughout this guide ConventionsDocumentation RelatedComments Pddtechpubscomments@3com.comRegistration ProductSwitch Features Page Switch Features What isManagement Software?Switch Features Overview For more information about multicast filtering, see Using Ports operating at 1000 Mbps only support full duplex modeDuplex Flow ControlResilience Features For more information about STP and RSTP, see UsingEvent Notification Quality of ServiceFor more information about Quality of Service, see Using For more information about VLANs, see Setting Up Virtual Port Features Ports operating at 1000 Mbps support full duplex mode only Port Features Switch units connected using an aggregated link Aggregated Links Traffic Distribution and Link Failure on Aggregated Links 2 Gbps aggregated link between two Switch units Optimizing Bandwidth Relevant multicast group Using Multicast FilteringUsing Multicast Filtering Query Mode Query modeSnooping Mode That support IP Igmp v2 protocolIgmp multicast filtering works as follows Enabling Igmp Multicast LearningIgmp Multicast Filtering Using Multicast Filtering Network Spanning TreeProtocol STP Ieee 802.1w standard What is STP? Traffic flowing through Bridge B Port Speed Link Type Path Cost 802.1D-1998 802.1w Using Resilience Features Port costs in a network Configuration 3 Redundancy for Cabling Error Configuration 1 Redundancy for Backbone LinkConfiguration 2 Redundancy through Meshed Backbone STP configurations Using STP on a Default BehaviorNetwork with Multiple VLANsConfiguration that separates VLANs Using Resilience Features How Switch What is the SwitchDatabase? Database EntriesDatabases entries can have three states Switch Database Entry StatesPrioritization? What is TrafficAvailable Converged network applications Used by organizations with a802.1D Priority Levels How Traffic Prioritization Works Traffic Prioritization Your SwitchConfiguring traffic prioritization for QoS on a Switch How traffic is processed to provide Quality of ServiceUsing Traffic Prioritization That allows you to monitor the traffic of LANs or VLANs What is RMON?Network efficiency and reduce the load on your network It improves your efficiency AlarmsEvents Rmon SwitchLow Threshold Statistic High Threshold Recovery Period Low Threshold Statistic High Threshold Recovery Status Monitoring and Statistics Hardware Status Monitoring Example device summary web Setting UP Virtual Lans VLANs provide extra security VLANs ease the movement of devices on networksVLANs help to control traffic Creating an Ieee 802.1Q Tagged Link MembershipTwo Vlans connected via a router Using Untagged ConfigurationConfigure the VLANs ExamplesAdd ports to the VLANs Configure the VLANs on SwitchAdd endstation ports on Switch 1 to the VLANs Add endstation ports on Switch 2 to the VLANs Add port 12 on Switch 1 to the VLANsAdd port 11 on Switch 2 to the VLANs Check the Vlan membership for both switchesConfiguration Obtains IP How Your SwitchInformation How Automatic IPHow Automatic IP Configuration Works Considerations Automatic IP configuration featureThat a Switch may change its IP address whilst in use IP Addressing Configuration RulesGlossary IndexPage Configuration Rules for Gigabit Ethernet Configuration RulesFast Ethernet configuration rules Configuration Rules for Fast Ethernet Appendix a Configuration Rules This chapter contains the following sections NetworkSimple Network Segments are connected to the Switch which is positionedFamily and 4900 family can be used in your network ExampleDual Speed Hub Switch Improving Advanced NetworkResilience of Your NetworkTherefore the overall network performance EnhancingPerformance Simple Overview IP AddressesIP Address Obtaining a Registered IP AddressNetwork Portion Dotted Decimal NotationDecimal value of an octet whose bits are all 1s is Subnet mask Extending the Network Prefix Subnet Mask NumberingOr leave the field blank Glossary Glossary Ethernet EndstationEthernet address Fast EthernetHalf duplex GatewayHub Ieee 802.1DLine speed See baud Glossary Rstp See Rapid Spanning Tree Protocol STP See Spanning Tree Protocol STP Page Glossary Numbers IndexIgmp multicast filtering 30 Internet Addresses 85 InterNIC Sub-networks. See subnets Switch Database

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