Editing IPv4 (section)

===First and last subnet addresses===
{{See also|IPv4 subnetting reference}}
The first address in a subnet is used to identify the subnet itself. In this address all host bits are ''0''. To avoid ambiguity in representation, this address is reserved.{{Ref RFC|923|quote=Special Addresses: In certain contexts, it is useful to have fixed addresses with functional significance rather than as identifiers of specific hosts. When such usage is called for, the address zero is to be interpreted as meaning "this", as in "this network".}} The last address has all host bits set to ''1''. It is used as a local [[broadcast address]] for sending messages to all devices on the subnet simultaneously. For networks of size {{IPaddr||24}} or larger, the broadcast address always ends in 255.

For example, in the subnet {{IPaddr|192.168.5.0|24}} (subnet mask {{IPaddr||24|netmask=dotted}}) the identifier {{IPaddr|192.168.5.0}} is used to refer to the entire subnet. The broadcast address of the network is {{IPaddr|192.168.5.255}}.

{|class=wikitable style=margin:left
!Type!!Binary form!!Dot-decimal notation
|-
|Network space
|<code>11000000.10101000.00000101.<span style=color:red>00000000</span></code>
|192.168.5.0 
|-
|Broadcast address
|<code>11000000.10101000.00000101.<span style=color:red>11111111</span></code>
|192.168.5.255
|-
|colspan=3|<small>In red, is shown the host part of the IP address; the other part is the network prefix. The host gets inverted (logical NOT), but the network prefix remains intact.</small>
|}

However, this does not mean that every address ending in 0 or 255 cannot be used as a host address. For example, in the {{IPaddr||16}} subnet {{IPaddr|192.168.0.0|16|netmask=dotted}}, which is equivalent to the address range {{IPaddr|192.168.0.0}}–{{IPaddr|192.168.255.255}}, the broadcast address is {{IPaddr|192.168.255.255}}. One can use the following addresses for hosts, even though they end with 255: {{IPaddr|192.168.1.255}}, {{IPaddr|192.168.2.255}}, etc. Also, {{IPaddr|192.168.0.0}} is the network identifier and must not be assigned to an interface.{{Ref RFC|1122|rp=31}} The addresses {{IPaddr|192.168.1.0}}, {{IPaddr|192.168.2.0}}, etc., may be assigned, despite ending with 0.

In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones.{{Ref RFC|1122|rp=66}}

In networks smaller than {{IPaddr||24}}, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet {{IPaddr|203.0.113.16|28}} has the broadcast address {{IPaddr|203.0.113.31}}.

{|class=wikitable style=margin:left
!Type!!Binary form!!Dot-decimal notation
|-
|Network space
|<code>11001011.00000000.01110001.0001<span style=color:red>0000</span></code>
|203.0.113.16
|-
|Broadcast address
|<code>11001011.00000000.01110001.0001<span style=color:red>1111</span></code>
|203.0.113.31
|-
|colspan=3|<small>In red, is shown the host part of the IP address; the other part is the network prefix. The host gets inverted (logical NOT), but the network prefix remains intact.</small>
|}

As a special case, a {{IPaddr||31}} network has capacity for just two hosts. These networks are typically used for point-to-point connections. There is no network identifier or broadcast address for these networks.{{Ref RFC|3021}}