Normally, long-term blocks of IP addresses or ranges do not affect logged-in editors. However, it is occasionally necessary to block both anonymous and logged-in users, using autoblock and/or hard blocks, to prevent disruption.
[...]
Editing via an anonymous proxy, including open proxies and VPN services, can be easily abused. In this case, IP block exemption is granted only to trusted users. Examples of editors who may reasonably request an exemption include users who show they can contribute to the encyclopedia, and existing users with a history of valid non-disruptive contribution.
A hard IP address block (account creation blocked, apply block to logged-in users from this IP address) disables all editing and account creation from behind the blocked IP address, whether they be attempted anonymously or using an account (with the exception of accounts that are IP-block exempt). This is typically used when the level of vandalism or disruption via creation of "throwaway" accounts is such that all editing from the IP address is to be prevented except after individual checking of requests. Open proxies are hard-blocked on detection, and Tor IP addresses are automatically blocked by the Tor block extension.
Open or anonymizing proxies, including Tor as well as many public VPNs, may be blocked from editing for any period at any time. While this may affect legitimate users, they are not the intended targets and may freely use proxies until those are blocked. No restrictions are placed on reading Wikipedia through an open or anonymous proxy.