Whether we usual use dot-decimal notation to represent network or IP by opposite mask? # a network  # a IP


if we represent a network, we can use, but how can we represent a IP address of with mask? Is it not too?
I also sometimes find the can represent a network, what's the difference.


Some friend says like the can not stand for the networks, but in the route, there obviously regard this as a subnet. is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/0
L is directly connected, GigabitEthernet0/0/0 is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0/1
L is directly connected, GigabitEthernet0/0/1
  • 1
    I don't understand your question. A network mask will tell you which part of the address is the network, and which part is the host. Read both parts of this answer to see if your question is answered by something in the answers.
    – Ron Maupin
    Nov 15, 2018 at 2:12
  • Hello 244boy and welcome to NE. /24 is an alternative but equivalent method of writing the mask, which has 24 ones at the beginning followed by 8 zeros. is only used in ACLs, see my answer.
    – jonathanjo
    Nov 15, 2018 at 3:08
  • @jonathanjo Does the ospf need the
    – 244boy
    Nov 15, 2018 at 3:27
  • Yes, in Cisco configurations, OSPF uses wildcard bits, not masks. Answer below updated.
    – jonathanjo
    Nov 15, 2018 at 10:24

2 Answers 2


As far as I know, the only thing which uses the "opposite mask" are the "wildcard bits" of access control lists in Cisco equipment. These can represent hosts or networks (and portions or aggregations of networks), but in practice they don't represent hosts because they are optional on Cisco IOS:

ip access-list standard NTPCLIENTS
 permit   # network
 permit        # host
 permit                # host

Edit: ... also in Cisco configurations, some parts of the OSPF configurations also use wildcard bits, not masks. It needs saying that it doesn't mean anything different, it's just a different way of writing it, required for no reason other than that's the way Cisco decided to do it.

  • Yes OSPFv2 uses wildcard masks, but it is only to determine which interfaces will participate in OSPF, not for advertising networks (many people get confused by that). OSPFv3 corrects that by moving the configuration to the actual interfaces.
    – Ron Maupin
    Nov 15, 2018 at 16:22

You are mistaken as both the first two examples you provide are networks.

---Networks--- (dotted decimal)            (CIDR)     (wildcard mask)

Each of these examples provides the information that defines a range of IP addresses, specifically -

To indicate a single host or IP address, your mask or wildcard mask would have to indicate that only a single IP address was indicated. For example:

---Host--- (dotted decimal)              (CIDR)         (wildcard mask)
  • the still is a network, which only have one IP.
    – 244boy
    Nov 15, 2018 at 5:57
  • @244boy, a single IP does not constitute a network. To establish any sort of IP network, you need a minimum of two (typically four) IP addresses. A /32 is used to indicate a single host IP.
    – YLearn
    Nov 15, 2018 at 5:59
  • So, we can use ?
    – 244boy
    Nov 15, 2018 at 6:30
  • To indicate a single host? Yes. In routing, this would indicate a "host route" or the route to take to reach a single host.
    – YLearn
    Nov 15, 2018 at 6:33
  • Hi, friend, check my update-02 of my post.
    – 244boy
    Nov 15, 2018 at 10:09

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