Netmask

Q: How deeply does Acing the CCNA Exam: Volume 2 Advanced Networking and Security cover netmask?

Defines the netmask and interprets dotted-decimal, hexadecimal, and prefix-length forms, without deriving bitwise operations.

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

A netmask is a bit mask that separates the network portion of an IP address from its host portion.

A netmask separates the network portion of an IP address from its host portion. Hosts use it to determine whether a destination belongs to the local LAN or requires communication through the default gateway. section 4.1

Recognize netmasks in common host configuration and routing output:

255.255.255.0 is equivalent to /24 and identifies the 192.168.1.0 LAN for an address such as 192.168.1.226. section 4.3.2 section 4.3.3
A host receives its netmask alongside an IP address, default gateway, and DNS server; DHCP automates delivery of these settings. section 4.1
The netmask also appears in connected-LAN routes, such as 255.255.255.0 for 192.168.1.0. section 4.3.3

Reading a host configuration

In Linux output, `inet 192.168.1.226 netmask 255.255.255.0` identifies the host address and its /24 netmask. macOS may display the same mask as `0xffffff00`. section 4.3.2 section 4.3.3

jeremy@linuxpc ~ $ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.226  netmask 255.255.255.0  broadcast 192.168.1.255    #1
        inet6 fe80::5054:ff:fe17:7862  prefixlen 64  scopeid 0x20<link>
        ether 52:54:00:17:78:62  txqueuelen 1000  (Ethernet)                  #2
. . .
jeremy@linuxpc ~ $ route
Kernel IP routing table
Destination   Gateway       Genmask        Flags  MSS  Window irtt Iface
0.0.0.0       192.168.1.1   0.0.0.0        UG     0    0      0    eth0       #3
192.168.1.0   0.0.0.0       255.255.255.0  U      0    0      0    eth0       #4
. . .
#1 linuxpc’s IP address is 192.168.1.226/24.
#2 linuxpc’s MAC address is 52:54:00:17:78:62.
#3 A default route
#4 A route to linuxpc’s connected LAN

Inspect annotated Linux interface and routing output showing the netmask, broadcast address, connected LAN, and default route. section 4.3.3

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

Cover of Acing the CCNA Exam: Volume 2 Advanced Networking and Security

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Also explained in

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Acing the CCNA Exam: Volume 1 Fundamentals and ProtocolsA netmask is a 32-bit value paired with an IPv4 address to identify which address bits belong to the network portion and which belong to the host portion.

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Related concepts

classless inter-domain routingused by · Classless Inter-Domain Routing is an IP addressing and routing scheme that represents networks and address ranges with a network prefix and prefix length.
prefix lengthequivalent to · A prefix length is the number of leading bits in an IP address that define its network portion, conventionally expressed in slash notation.
wildcard maskcontrasts with · A wildcard mask is a 32-bit bit mask in which zero bits require corresponding address bits to match and one bits ignore them during address matching.
network portionmeasures · The network portion is the part of an IP address, determined by its prefix or subnet mask, that identifies the network or subnet containing the addressed host.
IP addressconfigures · An IP address is a numerical address used by the Internet Protocol to identify a network interface and support packet delivery across networks.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 2 Advanced Networking and Security. The section links point to the parts of the book it draws on.